You want a source value that corresponds to a given month to show in the report? Then which would you pick for 08/01/2015?
Basically, if your report performs aggregate functions on data (such as Sum, Count, Avg, etc), it's not possible to show separate values for a field on which you do not perform an aggregate function. You would have to show the source values in the detail section of your report as separate transactions and put the aggregate values in a sub report or report footer.
Or am I not understanding the problem?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.