Pivot Table groups & cumulative totals
 

I have a list of transactions over several years. I need to group the
transactions by year and month to show those totals, but also run a
calculated field cumulative total over all the years. I am using a pivot
table so I can show these totals for different vendors and product types.

My problem: when I group the data, each year starts with zeros and my
calculated field doesn't calculate properly. I get the proper cumulative
total for each year, but I need it to go over all years (all groups). Any
way to do this?

Example - PT looks like this:

Year 1 item 1 + 8.00
item 2 - 6.00
cum. total + 2.00
Year 2 item 1 + 7.00
item 2 - 4.00
cum. total + 3.00 (should be +5.00)

Thank you!