I have a few columns. The month and year are two separate groups in the report, but are also posted in the details section.
Group 1 is Year, Group 2 is Month
Here's how the design view looks (roughly):
Group Header 1 - year
Group Header 2 - @month (formatted so the month displays as "01-JAN", "02-FEB", etc.)
Details a - name, month ("mm"), year
Details b - baseline dollars, actual billed, funded dollars
Group Footer 2 - Sum of Baseline Dollars, Sum of Actual Billed, Sum of Funded Dollars
The chart is displaying running total fields of: baseline dollars, actual billed, and funded dollars.
I tried using your formula using the formula suggestion, but CR didn't like the string type.