I have just finished replacing my Python plotters (using the ancient Visual Python & the more modern Pi3D) with OpenGL using c99/FreeGLUT. The three plotters are based on the Solar.c example, but with calls to gravitational simulations in c99 replacing the “clockwork” innards.
FreeGLUT seems to have all the features I need, and no more.
If anyone can give pointers to improving the GL part I welcome feedback - If anyone can improve the simulations themselves my jaw will drop, but I welcome that too 
https://github.com/m4r35n357/ODE-Playground/tree/pure_c
[EDIT] BTW I am not a pro and this is open source