Graphics library in C

I like the Cairo library. It has a nice interface to C and it can output in many formats.

To plot 2D and 3D graphs in C I would recommend the library DISLIN. You can see examples here or there.

The code is pretty easy to use and gives nice results.

This question is a bit vague, "graphics" is a wide field. You can get pretty far using just plain SDL, but it might also be considered "too low-level". You need to provide more requirements.