Display Cumulative Values in Line Chart When "Drilling"

[#1]

I have created a line chart that shows the cumulative hours spent month by month over the life of a task.

When the user selects a monthly data point (like May 2016 in the below image), the line chart displays the activity for that selected month.

The problem is, because I'm excluding all of the previous months, the accumulation restarts at zero and plots up to the last accumulated value in that month.  What I need is to have the month start where all of the previous months ended. (i.e. If January "units" ended at 150 and there are 50 "units" in February, February should start at 150 and end at 200, not start at 0 and end at 50.)

I'm certain the answer lies in Set Analysis, but I have tried and tried and can't figure out how to write the expression.

Any help is greatly appreciated.  Thank you in advance.