I have used RJSGraph (http://www.rjsweb.net/) graphing software to plot on log/log axis.
The software runs on Windows and is free. I use Excell to generate the numbers for the Y-Axis and cut&pasted them in to RJS Graph.
Example Plot here (I tried scrunching it down to fit APUG but it lost too much resolution).
[Edit: Spelt Pat's name right this time...]
I'd be interested if they fit other's curves just to make sure I have not boo-booed too much...