Show montly variation in percent

Hi all,

Im trying to create a report that shows an increase/decrease in percent pr. month

I got a cross-tab set up which shows the amount of Service Calls received pr. month, arranged in a row based on the Service they are registered on.

What i'm trying to do is to instead of showing the amount of calls, is to show the variation in percent between each month.

Cant seem to figure out how to set this up, anyone got any pointers?

Thanks,

Frode