QtGraph is a project mainly written in C++ and C, it's free.
A simple plotter (School project)
QtGraph is graph builder (or plotter) project based on Qt toolkit.
For know it can plot many functions in good quality quite fast (0-20 seconds).
Supported functions: abs, sin, cos, tan or tg, ctan or ctg, floor, sqrt, arcsin, arccos, arctan or arctg And one other just for fun :)
Supported constants: pi
How-to build: Just use QtCreator from Qt SDK
On Linux (Debian/Ubuntu):
$sudo aptitude install qt-sdk
On Windows:
download from http://qt.nokia.com/downloads