Matrix: percentage growth same period previous year

I'm trying to creating the following matrix in SSRS 2008R2 (with more product categories than shown here)

The matrix only shows the last two year per product.

The matrix in the report builder looks like this:

For me it's unclear what the expression should be. (or perhaps I need to make a calculation in my TSQL?)

As Row Groups in the matrix I've got: Product; Year

As Column Groups i've got Month

I can't get it to work. Previous doesn't seem to work in this case (only when I got a total of each year (e.g. one column instead of 12))

Can someone please help me out?

I really need a solution for this, as I need to create this type of matrix more often.