Earlier versions of Octave provided plotting through the use of gnuplot. This capability is still available. But, a newer plotting capability is provided by access to OpenGL. Which plotting system is used is controlled by the graphics_toolkit function. See Graphics Toolkits.
The function call graphics_toolkit ("qt") selects the Qt/OpenGL system, graphics_toolkit ("fltk") selects the FLTK/OpenGL system, and graphics_toolkit ("gnuplot") selects the gnuplot system. The three systems may be used selectively through the use of the graphics_toolkit property of the graphics handle for each figure. This is explained in Graphics Data Structures. Caution: The OpenGL-based toolkits use single precision variables internally which limits the maximum value that can be displayed to approximately 10^{38}. If your data contains larger values you must use the gnuplot toolkit which supports values up to 10^{308}.
© 1996–2018 John W. Eaton
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https://octave.org/doc/interpreter/Introduction-to-Plotting.html