You should be able to accomplish this using a Level of Detail calculation and then plot that on your dual axis. If you have a sample workbook to attach, that would be very helpful.
That was my thought too. I tried using the EXCLUDE function against the [week date] which works for filtering on one month but when i filter for 2 months it calculates the duration for both months. I want a month to month calculation (per pane). is that feasible?
thinking of just developing in the SQL which drives the data
If you can attach a workbook I'm happy to take a closer look.