In the development of WikiPlot, which is getting really close to a release, we have written some classe for plotting mathematical expressions using php. Now I have finally be able to relase a public test version of the plotting class:
Jopsen.dk/plot/expression
Where expression is a mathematical expression, like 2x^2+4sin(3x) or what ever sick expression you may find 
Anyway it’s the internal plotting class for WikiPlot, and you can try out…