FAQ
This is a list of frequently asked questions (FAQ) for GNU Octave users.
We are always looking for new questions (with answers), better answers, or both. Feel free to edit this page with your changes.
Where do I get additional help?
If you can't find an answer to your question in this FAQ, wiki, or in the manual (PDF) you can:
 Search for an answer in our mailing list archives
 Contact our user community using our Octave Discourse.
 Contact our user community using our IRC chat room
#octave
in Freenode
General
What is Octave?
GNU Octave is a highlevel interpreted language, primarily intended for numerical computations. It provides capabilities for the numerical solution of linear and nonlinear problems, and for performing other numerical experiments. It also provides extensive graphics capabilities for data visualization and manipulation. GNU Octave is normally used through its interactive interface (CLI and GUI), but it can also be used to write noninteractive programs. The GNU Octave language is quite similar to Matlab so that most programs are easily portable.
The GNU Octave distribution includes a 1000+ page Texinfo manual. Access to the complete text of the manual is available via the doc
command at the GNU Octave prompt.
What is Octave Forge?
Octave Forge is a collection of packages for GNU Octave, something similar to the Matlab toolboxes. When talking about the two projects at the same time, GNU Octave is usually referred to as Octave core (or just "core"). Octave Forge also serves as a test bed for code that may eventually end up in the core, and distributes binaries for systems with a lack of developers tools (mainly Windows).
Who uses Octave?
A huge number of people ranging from students to researchers involved in various fields such as statistics,Machine Learning, data analytics, etc. Universities use it for research and teaching, companies of all sizes for development and individuals for certain private purposes. See Who Uses Octave? for more clarity.
Who develops Octave?
Discussions about writing the software that would eventually become Octave started in about 1988 with James B. Rawlings and John W. Eaton at the University of Texas. John W. Eaton is the original author of Octave, starting fulltime development in February 1992. He is still the primary maintainer. The community of users and developers has in addition contributed some code and fuels the discussion on the mailing lists help@octave.org (user forum), maintainers@octave.org (development issues). Since 2011, GNU Octave regularly participates in Summer of Code events as a mentor organisation. As part of those events, several students contributed to Octave by helping in removal of bugs and development of new features.
Why "Octave"?
Octave's name has nothing to do with music. It is named after Octave Levenspiel, a former professor of John who was famous for his ability to do quick backoftheenvelope calculations. You can hear John pronounce the name "Octave" a few times in this video. We hope that GNU Octave will help perform computations with the same ease as Dr. Levenspiel.
Why "GNU" Octave?
The GNU Project was launched in 1984 to develop a complete Unixlike operating system which is free software: the GNU system. GNU is a recursive acronym for "GNU's Not Unix"; it is pronounced g'noo.
The Free Software Foundation (FSF) is the principal organization that has sponsored the GNU Project.
Octave became GNU Octave in 1997 (beginning with version 2.0.6). This meant agreeing to consider Octave a part of the GNU Project and support the efforts of the FSF. A big part of this effort is to adhere to the GNU coding standards and to benefit from GNU's infrastructure (e.g. code hosting and bug tracking). Additionally, Octave receives sponsorship from the FSF's Working Together fund. However, Octave is not and has never been developed by the FSF.
How can I cite Octave?
Octave is free software and does not legally bind you to cite it. However, we have invested a lot of time and effort in creating GNU Octave, and we would appreciate if you would cite if you used. To cite GNU Octave in publications use:
John W. Eaton, David Bateman, Søren Hauberg, Rik Wehbring (2021). GNU Octave version 6.3.0 manual: a highlevel interactive language for numerical computations. URL https://www.gnu.org/software/octave/doc/v6.3.0/
A BibTeX entry for LaTeX users is:
@manual{, title = {{GNU Octave} version 6.3.0 manual: a highlevel interactive language for numerical computations}, author = {John W. Eaton and David Bateman and S{\o}ren Hauberg and Rik Wehbring}, year = {2021}, url = {https://www.gnu.org/software/octave/doc/v6.3.0/}, }
Run citation
at the Octave prompt for details on how to best cite the Octave version you are running. Certain Octave packages also have recommended citations in which case use citation package_name
.
Note that there are two reasons for citing the software used. One is giving recognition to the work done by others which we have already addressed to. The other is giving details on the system used so that experiments can be replicated. For this, you should cite the version of Octave and all packages used (you can get this information using the ver
command), as well as any details of your setup as part of your Methods. In addition, you should make your source available as well. See How to cite and describe software for more details and an in depth discussion for the same.
What documentation exists for Octave?
Besides this wiki, the GNU Octave distribution includes a 1000+ page Texinfo manual (PDF). Access to the complete text of the manual is available via the doc
command at the GNU Octave prompt. If you have problems using this manual, or find that some topic is not adequately explained, indexed, or crossreferenced, please report it on http://bugs.octave.org.
How can I report a bug in Octave?
Please read our website http://www.octave.org/bugs.html.
Common problems
Octave does not start
The following steps have been the solution to several bug reports and help requests. Please try them before asking for further support. If nothing below helps, please give us the following information:
 Operating system: e.g. MS Windows 10 (version 2004) or Ubuntu 20.04
 GNU Octave version: e.g. Version 6.3.0
 Installation method: e.g. Downloaded and installed "octave6.3.0w64installer.exe" from https://www.octave.org/download.html
MS Windows
 After Octave upgrade the GUI does not open / shuts down immediately.
 Solution: Delete the folder C:\Users\YOUR_USER_NAME\.config\octave
 Missing/conflicting files.
 Solution: Remove/Uninstall all existing Octave versions. Restart the system. Install GNU Octave again.
 Permission errors.
 Solution 1: Consult your malware detection (a.k.a. AntiVirus) software, if files are blocked.
 Solution 2: Did you install Octave on a networkdrive? Do you have the execution permissions?
 Solution 3: Is your computer managed by your company? Does your administrator prohibit script execution?
I do not see any output of my script until it has finished?
By default Octave is set to pass its screen output through a pager (usually the default pager is "less") which allows things such as navigating through the output with arrow keys or searching for text or regular expressions within the output. The pager only displays the output after it's finished receiving it, so when it is active you'll not be able to see anything until your script has terminated. To change this behavior temporarily or permanently you may want to use one of the options described in the manual.
When I try plotting from a script, why am I not seeing anything?
If you are running an Octave script that includes a plotting command, the script and Octave may terminate immediately. So the plot window does show up, but immediately closes when Octave finishes execution. Alternatively, if using fltk, the plot window needs a readline loop to show up (the time when Octave is sitting around doing nothing waiting for interactive input).
A common solution is to put a pause
command at the end of your script.
How do I get sound input or output in MS Windows?
Sound input from a sound card and output to a sound card is fully supported since Octave 4.0.0 for all platforms. If you have problems with the audio I/O functions using Octave 4.0.0 or a newer version, please report them on the bug tracker.
I have problem X using the latest Octave version
Please be more specific about what you mean by "latest version"?
 The latest stable version is 6.3.0. Be aware that you may still have an older version due to whatever distribution method you are using. To get a newer stable version for your system see the following as in accordance to your Operating system: GNU/Linux, macOS, or Windows.
 If you refer to the latest Mercurial revision, please specify the changeset ID not the revision number, e.g. the output of
hg summary
orhg id
. Changeset IDs are globally unique across all repos.
If your problem still persists with the "latest version", then please report a bug or ask for help at help@octave.org. Otherwise, don't be surprised if volunteers are less inclined to help you with a problem that only exists in an older version of Octave and is already fixed in a newer version.
Why is Octave's floatingpoint computation wrong?
Floatingpoint arithmetic is an approximation in binary to arithmetic on real or complex numbers. Just like you cannot represent 1/3 exactly in decimal arithmetic (0.333333... is only a rough approximation to 1/3), you cannot represent some fractions like exactly in base 2. In binary, the representation to one tenth is where the bar indicates that it repeats infinitely (like how in decimal). Because this infinite repetition cannot be represented exactly with a finite number of digits, rounding errors occur for values that appear to be exact in decimal but are in fact approximations in binary, such as for example how 0.3  0.2  0.1 is not equal to zero.
In addition, some advanced operations are computed by approximation and there is no guarantee for them to be accurate, see Tablemaker's dilemma for further references. Their results are systemdependent.
This isn't a bug that is limited to GNUOctave & it happens with any program that uses IEEE 754 floatingpoint arithmetic. To be fair, IEEE 754 also specifies decimal floatingpoint arithmetic, which has never seen wide adoption. The reason why Octave and other programs using IEEE 754 binary floatingpoint numbers is that they are fast, because they are implemented in hardware or system libraries. Unless you are using very exotic hardware, Octave will use your computer's processor for basic floatingpoint arithmetic.
Another approach to deal with rounding errors is interval arithmetic with the Interval package or symbolic computations with the Symbolic package. These approaches are likely to be slower, since not all operations can be performed on Hardware like pure floatingpoint arithmetic.
To learn more about floatingpoint arithmetic, consult this Wikipedia article or the classical reference by David Goldberg What Every Computer Scientist Should Know About FloatingPoint Arithmetic.
Missing lines when printing under Windows with OpenGL toolkit and Intel integrated GPU
Some windows users with integrated Intel GPUs have reported missing lines when printing with an OpenGL toolkit like FLTK or Qt. #42534
Users with this kind of problem should try to install/update their Intel OpenGL drivers for Windows or consider installing Mesa drivers from http://qtproject.org/wiki/CrosscompilingMesaforWindows.
See also https://www.opengl.org/wiki/FAQ#Why_is_my_GL_version_only_1.4_or_lower.3F .
Plot hangs and makes the GUI unresponsive
If the Qt graphics toolkit is used and "plot" is used for the first time, the fontconfig scanner searches the font directory to build a font cache. This can take up to 3min on slow CPUs. See #45458
Error message about invalid call to script or invalid use of script in index expression
If Octave shows an error message about invalid call to script .../close.m
or invalid use of of script .../close.m in index expression
, it means that you have created a script called close.m that is overriding the builtin Octave function close
. Octave functions and scripts share the same global namespace. It is best to avoid creating your own scripts or functions that have the same name as an Octave function as to avoid this error regarding the invalid call to script or invalid use of script in index expression.
Licensing issues
If I write code using Octave do I have to release it under the GPL?
The answer depends on precisely how the code is written and how it works:
 Code written entirely in the scripting language of Octave (interpreted code in .m files) may be released under the terms of whatever license you choose.
 Code written using Octave's native code interface (also known as a .oct file) necessarily links with Octave internals and is considered a derivative work of Octave. Therefore it must be released under terms that are compatible with the GPL.
 Code written using Octave's implementation of the Matlab MEX interface may be released under the terms of whatever license you choose, provided that the following conditions are met:
 The MEX file may not use any bindings that are specific to Octave, it has to use the MEX interface only. In other words, it should be possible in principle to use the MEX file with other programs that implement the MEX interface (e.g., Matlab). For example including an Octave header file or calling an Octave function within the MEX file, that is not related with Octave's implementation of the MEX interface make the MEX file a derivative work of Octave and has therefore to be released under terms that are compatible with the GPL.
 The MEX file may not be distributed together with Octave in such a way that they effectively create a single work. For example, you should not distribute the MEX file and Octave together in a single package such that Octave automatically loads and runs the MEX file when it starts up. There are other possible ways to effectively create a single work.
 Code that embeds the Octave interpreter (e.g., by calling the
octave_main
function), or that calls functions from Octave's libraries (e.g., liboctinterp, liboctave, or libcruft) is considered a derivative work of Octave and therefore must be released under terms that are compatible with the GPL.
Will you change the license of the Octave libraries for me?
No. Instead of asking us to change the licensing terms for Octave, we recommend that you release your program under terms that are compatible with the GPL, this way the free software community can be benefited from your work the same as you were/have benefited from the work of all the people who have contributed to Octave.
Should I favor the MEX interface to avoid the GPL?
No. The original reason for implementing the MEX interface for Octave was to allow Octave to run free software that uses MEX files (the particular goal was to run sundialsTB in Octave). The intent was to liberate that software from Matlab and increase the amount of free softwares available to Octave users & not to enable people to write proprietary code for Octave. For the good of the community, we strongly encourage users of Octave to release the code they write for Octave under terms that are compatible with the GPL.
Why can't I use code from File Exchange in Octave?
According to the Matlab Central Terms of Use (Last updated: 10Aug2016), all submitted code is licensed under the BSD license by default (cf. § 5 Terms of Use), but it is clearly stated that:
Content submitted to File Exchange may only be used with MathWorks products.—§ 2(a)(iii) Terms of Use
That does not apply to GNU Octave, therefore the usage is in general prohibited. It should suffice — although interpretations of this vary — to contact the author directly to send you the code personally (maybe released under a free license), or download the code from the author's own website, if available. Some examples of letters/email sent to authors for that purpose.
Installation
How can I install Octave on Windows?
 So as to install GNUOctave for Windows O.S, refer to : Octave for Microsoft Windows
How can I install Octave on MacOS?
 So as to install GNUOctave for MacOS, refer to : Octave for macOS
How can I install Octave on GNU/Linux?
 So as to install GNUOctave on GNU/Linux, refer to: Octave for GNU/Linux
How to install Octave on Android OR What is the Octave app available in the Google Play store?
There is an unofficial Octave app available for Android in the Google Play store. Please see Octave for Android for further information.
How can I install Octave on platform X?
Octave currently runs on GNU/Linux, macOS, and Windows. It should be possible to make Octave work on other systems as well. If you are interested in porting Octave to other systems, please contact the maintainers development mailing list maintainers@octave.org.
What Octave version should I use?
For general use, it is recommended to use the latest stable version of Octave (currently 6.3.0), available from http://www.octave.org/download.html. For development and bleedingedge features one can obtain the development source code from the Mercurial repository https://hg.savannah.gnu.org/hgweb/octave/graph/.
The used version of Octave is available via the ver
command and a list of uservisible changes since the last release is available via the news
command at the GNU Octave prompt.
On what platforms does Octave run?
Octave runs on any platform you can compile it on. Binary distributions exist for GNU/Linux, macOS, and Windows. To work fully functional, Octave requires the used platform to support the underlying numerical libraries like BLAS, LAPACK, SuiteSparse, etc., and for plotting OpenGL or gnuplot.
How can I obtain Octave's source code?
The latest version of the Octave source code (and older versions) is available from:
Since Octave is distributed under the terms of the GPL, you can get Octave from a friend who has a copy.
How can I build Octave from the source code?
To use Octave it is usually not required to build it from it's source code. Binary distributions exist for GNU/Linux, macOS, and Windows.
If you have reasons to build Octave from the source code, see Building for more information.
What do I need to build Octave from the source code?
For a list of build dependencies, refer to Building.
Do I need GCC to build Octave from the source code?
No. The development is done primarily with GCC, so you may hit some incompatibilities. Octave is intended to be portable to any standard conforming compiler (for example clang is known to work as well). If you face any difficulties that you think are bugs, please report them to the bug tracker, or ask for help on the help@octave.org mailing list.
What's new in Octave?
Each new Octave release introduces many new features. A complete list of user visible changes can be seen by running news
at the Octave prompt.
 What's new in the next version of Octave?
 See the NEWS file on the development branch.
 What was new in Octave Version X.Y.Z?
 See Release History.
Packages and Octave Forge
How do I install or load all Octave Forge packages?
Do not do it! Really, there is no reason to do this. Octave has many packages for different needs and is unlikely that you need all of them. You either have a small set of required packages, in which case you know them by name; or you want them all "just because", in which case you don't really need them.
The common misconception is that the more packages one has installed and loaded, the more complete and powerful its Octave installation will be. However, in the same way one would never install all perl modules, ruby gems, python packages, and C++ libraries (because it simply makes no sense), one should not install all Octave packages.
Packages should be installed and loaded selectively. Note that some packages are meant to shadow core functions changing the way Octave works, and that different packages can have different functions with the same name leading to unpredictable results.
If you really really really want to do load all packages, you can with the following:
## WARNING: loading all packages is probably not the solution you are looking for.
cellfun (@(x) pkg ("load", x.name), pkg ("list"));
I have installed a package but still get a "foo undefined" error?
You have probably forgotten to load the package. Use pkg load packagename
to load it. Most packages are no longer loaded automatically to avoid surprises. See reasoning on related FAQ how do I install all Octave packages. If you want a specific package to be loaded by default at startup, consider adding the pkg load
command on your .octaverc file.
I cannot install a package. Octave complains about a missing mkoctfile.
You should normally use your distribution's packages. For Debian and Fedora, Octave package foo
will be a deb or rpm called octavefoo
, and you should install that instead using apt
or yum
.
If you really need to build Octave packages from source to install them, you'll need mkoctfile
. Most distributions split Octave into several packages. The script mkoctfile
is then part of a separate package:
 Debian/Ubuntu: liboctavedev
 Fedora:
octavedevel
How do I automatically load a package at Octave startup?
When Octave starts, it runs the file ~/.octaverc (in your user's home directory). If you want Octave to automatically load a package, simply add a pkg load pkgname
command to it. If the files does not exist, create it.
If you do this, remember that other people may not have Octave configured to load packages at startup. Therefore, if you write code for others, remember that your programs still need to load the packages they require.
Octave usage
How do I execute an Octave script?
First of all, make sure you understand the difference between script files and function files. If you want to execute a function defined in a file, just call the function like any other Octave function: foo(arg1, arg2);
To execute a script from within Octave, just type its name without the .m
extension. Thus, if you have a script called foo.m
, just type foo
from within the Octave command prompt to execute it. You have to make sure that the script is in your current working directory or in Octave's load path. Type pwd
to get the current working directory or type path
to see which paths belong to Octave's load path. The current working directory is referred to as "." in the output of path
.
If the script name has characters that are not valid for an Octave identifier, or if you do not want to use addpath
to add the script's location to the current path, you can use the run
function instead:
run ("Script Name With Spaces.m")
run ("/opt/local/foo.m")
An alternative is to run the script from outside Octave by calling Octave from your operating system shell. Unlike calling the script from inside Octave, this also allows you to pass arguments from the shell into the script, which the script can access using the argv
command:
$ octave thescript.m arg1 arg2
In a Unix environment, if the script has a shebang (e.g. #!/usr/bin/octave
) and executable permissions, you can call it like any other Unix program with arguments:
$ ./thescript arg1 arg2
If you call the script from the shell and it's plotting, please note how to plot when running a script from the shell.
How do I close a figure?
To close the current figure type close
in the Octave command prompt.
How do I set the number of displayed decimals?
You can control the number of displayed decimals using the format
command:
>> format long
>> pi
pi = 3.14159265358979
>> format short
>> pi
pi = 3.1416
How do I call an Octave function from C++?
Please read the manual https://www.gnu.org/software/octave/doc/interpreter/CallingOctaveFunctionsfromOct_002dFiles.html.
How do I change color/line definition in gnuplot postscript?
Here is a awk script to get a rainbow color map
#!/bin/awk f BEGIN { split("0 4 6 7 5 3 1 2 8", rainbow, " "); split("7 3 1 0 2 4 6 5 8", invraim, " "); } $1 ~ /\/LT[08]/ { n = substr($1, 4, 1); if (n == 0) lt = "{ PL [] 0.9 0.1 0.1 DL } def"; else if (n == 1) lt = "{ PL [4 dl 2 dl] 0.1 .75 0.1 DL } def"; else if (n == 2) lt = "{ PL [2 dl 3 dl] 0.1 0.1 0.9 DL } def"; else if (n == 3) lt = "{ PL [1 dl 1.5 dl] 0.9 0 0.8 DL } def"; else if (n == 4) lt = "{ PL [5 dl 2 dl 1 dl 2 dl] 0.1 0.8 0.8 DL } def"; else if (n == 5) lt = "{ PL [4 dl 3 dl 1 dl 3 dl] 0.9 0.8 0.2 DL } def"; else if (n == 6) lt = "{ PL [2 dl 2 dl 2 dl 4 dl] 0.5 0.3 0.1 DL } def"; else if (n == 7) lt = "{ PL [2 dl 2 dl 2 dl 2 dl 2 dl 4 dl] 1 0.4 0 DL } def"; else if (n == 8) lt = "{ PL [2 dl 2 dl 2 dl 2 dl 2 dl 2 dl 2 dl 4 dl] 0.5 0.5 0.5 DL } def"; $0 = sprintf("/LT%d %s", rainbow[n+1], lt); ##$0 = sprintf("/LT%x %s", invraim[n+1], lt); ##$0 = sprintf("/LT%x %s", n, lt); } { print; }
How do I tell if a file exists?
One can use the function exist
to tell if a regular file, say foo.txt
exist in Octave's load path, or the current directory:
>> exist ("foo.txt", "file") # 2, if file exists, 0 otherwise
ans = 2
How do I create a plot without a window popping up (plot to a file directly)?
figure (1, "visible", "off");
plot (sin (1:100));
print deps "/tmp/sin.eps"
One can set that behavior as default:
set (0, "defaultfigurevisible", "off");
How do I increase Octave's precision?
Octave's default numerical type is IEEE 754 binary64, a.k.a. "double" or "hardware floats". This type has a precision of 53 bits or about 16 decimal digits. It is supported by each modern computer hardware, so it is really fast. This type is assumed throughout for Octave's calculations. If more precision was required, one can obtain a "few bits more" by using integer types, e.g. uint64
, but in general one cannot expect more precision from any fast numerical software. Just to visualize "how big" those numerical limits are, consider the following table:
intmax ("uint64")

18,446,744,073,709,551,615

2^641

intmax ("int64")

9,223,372,036,854,775,807

2^631

flintmax ("double")

9,007,199,254,740,992

2^53

flintmax ("single")

16,777,216

2^24

When working with other types than "double" in Octave, one has to make sure, that the first operand is converted to the desired type:
>> uint64 (2^53 + 1)
ans = 9007199254740992
>> uint64 (2^53) + 1
ans = 9007199254740993
Notice the difference, in the first line the addition within the brackets is performed using double precision, therefore the result gets "truncated" to the maximum possible value without a warning. The third line uses throughout uint64 precision.
Consider carefully if your problem really needs more precision. Often if you're running out of precision the problem lies fundamentally in your methods being numerically unstable, thus more precision will not help you here.
If you absolutely must have more precision, you're at present better off using a CAS instead of Octave. However, CAS or symbolic computations must be implemented in software which makes it much slower than hardware floats. An example of such a CAS is Sage or have a look at Octave's Symbolic package.
How do I run a Matlab Pfile in Octave?
You can't. Matlab Pfiles (files with a .p
file extension), also known as Pcode, are obfuscated files that cannot be run outside of Matlab itself. The original source Matlab mfiles that were used to generate these Pfiles should be used in Octave instead.
There are no plans to support running Pfiles produced by Matlab in Octave.
How does Octave solve linear systems?
In addition to consulting Octave's source for the precise details, you can read the Octave manual for a complete highlevel description of the algorithm that Octave uses to decide how to solve a particular linear system, e.g. how the backslash operator A \ x
will be interpreted. Sections Techniques Used for Linear Algebra and Linear Algebra on Sparse Matrices from the manual describe this procedure.
How do I do X?
You are probably looking for the function lookfor
. This function searches the help text of all functions for a specific string and returns a list of functions. Note that by default it will only search the first line of the help text (check help lookfor
at the octave prompt for more). The following example helps to find the function to calculate correlation coefficient in a matrix:
>> lookfor correlation corr Compute matrix of correlation coefficients. corrcoef Compute a matrix of correlation coefficients. spearman Compute Spearman's rank correlation coefficient RHO.
Also, there's a high chance that the function name has a suggestive name, and so you can try autocompletion to get some hints. For the previous example, typing corr
at the octave prompt followed by pressing the Tab ↹Key twice would suggest the following:
>> corr corr corrcoef
Differences between Octave and Matlab
GUI
Does Octave have a GUI?
Yes! It was officially released with Octave 4.0.0. It was also available since version 3.8.0 as an experimental feature (use the forcegui
option to start Octave).
Why did you create yet another GUI instead of making one that already exists better?
The previously existing GUIs were not part of Octave itself. The integration within Octave was rather bad, as all of them treated Octave as a foreign black box and used pipes for communication. This approach is bound to fail with each new version of Octave, as any fix would only be temporary. For historical reasons and to honor the approaches, a short list of previous GUIs for Octave:
 QtOctave was a great, beautiful, and very useful tool which is now abandoned and incompatible with newer versions of Octave. We are thankful to its developers to make it free software so we could reuse large chunks of it for what is now the Octave GUI.
 Quint was a project for an Octave GUI that actually tried to do it right. Eventually it was merged into the Octave repository and is no longer a separate project. Also, many bits from QtOctave were reused in the GUI.
 Xoctave, which is proprietary and commercial.
 GUI Octave, which was proprietary and is no longer available.
Graphics: backends and toolkits
What are the supported graphics backends?
 OpenGL via the graphics toolkits qt (current default) and fltk
 gnuplot via the gnuplot graphics toolkit
How do I change my graphics toolkit?
There are three commands to deal with graphics toolkits:
available_graphics_toolkits

lists all available graphics toolkits 
graphics_toolkit

displays the currently used graphics toolkit 
graphics_toolkit ("qt/fltk/gnuplot")

sets the graphics toolkit to either of qt, fltk, or gnuplot, if available 
Why did you replace gnuplot with an OpenGL backend?
The development of Octave is committed to being both compatible with Matlab and adding additional features. Toward those ends, the developers decided to introduce a native OpenGL backend that supports Matlab handle graphics and its uicontrols. Starting with the 3.8 release, Octave uses OpenGL graphics by default (with FLTK widgets in Octave 3.8 and Qt widgets in Octave 4.0 and later).
Are there any plans to remove the gnuplot backend?
No. There are no plans to remove the gnuplot backend. It will be available as long as our users find it useful.
How can I implement a new graphics backend/toolkit?
This is one of those times where the best documentation is to read the existing code. We have three different toolkits in Octave now, so there are some examples to draw from.
Development
When will feature X be released or implemented?
When it's ready, sooner if you help. You can send us patches if you can implement feature X yourself. If you can't, some developers may be convinced to work on your specific problem for some money.
How can I get involved in Octave development?
 See Developer FAQ.