Reporting Services matrix problem

Hello,

I have a report in SQL reporting services 2005 in which I am using a matrix. Basically, it dynamically generates two years (ie 2006 and 2007) as a column header. this works great. Problem I'm having is a want to do a change year over year in one column at the end:

example

Insurer 2006 2007
Insurer1 10 25
Insurer2 13 45
Insurer3 30 18


I can't figure out how to create my own column on a matrix which just calulates the change year over year. All that is available is a subtotal which is not what I want. Here's what I would like to see:

Insurer 2006 2007 Change
Insurer1 10 25 15
Insurer2 13 45 32
Insurer3 30 18 -12

I have to use a matrix because the year is based on user selection. If the years were not going to change, I would have just used a table instead of a matrix and hard coded the year. can something like this be done in a matrix?

Thanks,

Strick