This is a C++ X11 collection of widget code, with emphasis on 3D graph plotting, but also has lots of other features.
It doesn't create it's own shared library, so the examples can get a bit big, but they run quickly, and compile out of the box. Well worth a look.
It uses the xearth data to present a nifty rotating earth.
The License is similar to GPL, but prevents military use, and also prevents porting to MS platforms, so it isn't quite GPL.