Thank you, this puts me way closer to the solution I'm looking for. This seems to work while Date is the dimension. Is there a way to make this work so that a weekly or monthly view also produces the same values?
The bar for Sum(x) over Febuary 2017 should be Sum(x) from 2/1 - 2/28
The bar for Sum(y) over February 2017 should be Sum(y) from 1/29 - 2/25.
What I've tried is: Below(Sum(Aggr(Sum(y), Date)), -3)
But when I switch from Date to Month, the bar shows from 3 months ago instead of 3 days ago.
I feel like I'm confusing the order things take place in or what the functions Aggr/Below are actually doing. I expect Aggr to produce a virtual table at the date level, and Below to operate on that, but it's still operating on the displayed dimension or the actual table.