Don't use a quick filter to select the month. Use a table calc.
When you use a table calc, tableau doesn't discard un-selected months from the table. It just displays what the user selects, but leaves the underlying table intact.
Attached is an old 8.2 example I have for using LOOKUP() (which is a table calc) to select months along a running-sum line. You will notice that the running sum values remain intact even when things are de-selected. (Running sum is in the tooltip.)
LOOKUP question.twbx 369.0 KB