Sounds like a perfect case for set analysis...
Create a calendar table and then a variable called 'vReportMonthSelected'. Set this variable to be the report month selected by the user eg. = only(getfieldselections(ReportMonth)), then use an expression like:
I'm sorry, I can't share my application, but data source is excel similar to following image with actual and budget data.
I have 2 years of data: actual HC (full 2013 and Jan-May 2014) and budget HC (full 2013 and full 2014)
If the user selects Year = 2014 and Month: Jan|Feb|Mar ---> I need to show them sum (HC) just for Mar-2014