Half-life graph plot

[Caapi]

Hi,

I'd like to know if there is a web page or windows application available for plotting the half-life graphs for various substances, such that you can define doses and frequency. I think it is very handy to see how the plasma concentration changes over time, when it peaks etc. For now I have been using a freeware graph tool, but it is very cumbersome defining all the formulas by hand.
So anyone know where I can find such a tool?