Hi Snoopy28,
It is not yet possible to synthesise channels (which is what I guess you'd want to do to create new data from existing data via some function) - your best bet at the moment is to grab the data via WeatherCat's AppleScript interface (if you wanted to do it in real time) or alternatively in something like Excel after exporting the data.
[Edit: Note: Export in 1.0.x is
cra not very good, version 1.1 (currently in development) allows the export of everything]
WeatherCat's custom graphs (Tools->Custom Graphs) can plot up to 3 variables per graph in a variety of formats - examples at
http://live.trixology.com/custom/ (scroll down for the graphs).
Hope this helps,
Stu.