It seems it can only graph functions with the form y = f(x).
If you want something complete and simple to use - you don’t need a programming language, you already have more specialized tools like Desmos.
And if you want to do something really complex, and specialized tools are too specialized - code it yourself. I mean, in the worst case (if you have absolutely no idea specific to your graph) - you can always draw pixel-by-pixel.
Then, if it’s too slow - common mathematical optimizations for this case are trivial to Google. You can also look at an example of a fragment (per-pixel) shader in
C:\PABCWork.NET\Samples\StandardUnits\OpenGL и OpenCL\OpenGLABC\Точки на поле, in case you want to fully utilize GPU.