You can
put this solution on YOUR website!I'll assume you are a Windows user. You really should say though.
If you are comfortable with console style input then gnuplot is pretty good http://www.gnuplot.info/
Else, I've heard (but don't have any experience with) that GraphCalc http://www.graphcalc.com/ is pretty good.
GnuPlot is definitly free, I don't know about GraphCalc.