What I did one time to have an accumulative sum of sales base on the month of the selection was used the set analysis and some Variables:
I have a date island that include month, quarter, year etc and with that I make some variables like:
and I put a Selector for the month and the year in the report so the user can choose what to analyse, then in the report I used the next formula
where vMaxDay = max(Date) and vMinDay = =makedate(Y,1,1)
What i got was the accumulated sum of sales, if the user choose april the expression returns the sum of jan, feb, mar and apr... so Hope it helps.