Complex logic for trend chart.. Please help

[#1]

Hello,

I have install base data and cases data. Now, I want a chart of # of cases per install base.

Logic: If I want to compare for example Q1 2015 and Q2 2015 than in my numerator I want Total cases for Q1 and in my denominator total Install bases starting beginning till Q1 as one of my bar and for my second bar I want Total cases for Q2 in my numerator and in my denominator I want total Install bases till Q2.

I was able to achieve Install base accumulation per each quarter but I am not able to implement this logic in a chart.

If I select only one quarter than I can see correct result (Shown in IMG1), but if I choose two quarters to compare, my numbers are changed, here Q1 2015's numbers are correct and Q4 2014's numbers are changed/incorrect (Shown in IMG2)

Also one important thing to mention, this chart is based on two different dimensions. One for case creation quarter and one for install base installed quarter.

Any help will be appreciated.

Thank you,

Parth