If you include PrinProcMDName and FiscalYearMonth as fields in your Aggr() function (in addition to EntitityName and PatientAccount), when the user drills down by, e.g. FiscalYearMonth, on your chart, the current selections will be limited to the respective FiscalYearMonth. This will then be reflected in your aggregation i.e. the aggregation will only be performed over data for that FiscalYearMonth and your line should reflect this.
Does this work for you?