Calculating a cumulative total in a sumo card
I'm posting in ETL/Dataflow as I assume this is where the ultimate answer lies.
My end objective is I want to be able to present a simple table outlining my different customer cohorts (cohort is identified based on the month they placed their first order), and their average Gross Profit to us each month since joining.
Currently I can do this month by month, but I want to show this as a cumulative total too (ie per below, cumulative total for Period 2 is Period 1 plus Period 2 gross profit, then Period 3's cumulative total is period 1 plus period 2 plus period 3).
Ideally this is how I'd like it to look:
|March 2018||April 2018|
|Gross Profit||Cumulative Total||Gross Profit||Cumulative Total|
So far I have done a rank and window using magic ETL on gross profit each month by customer (and then a grouping on customer ID and month of order), however this falls down because not every customer orders each month, hence my cumulative total increases and decreases each month based on number of people in the cohort ordering, rather than being a constant increase as I would expect.
Any help our thoughts would be appreciated.