Making an accumulated sum graph with QuickSight

I was able to find an answer to this. I did it the following way:

runningSum(sum({count}), [truncDate("MM",{creationdate}) ASC], [{region}])

and I found an interesting catch.... whenever I tried adding at the moment that I'm configuring the data source, it'd fail.

So I went through and added the calculated field on the area where I can edit the visuals - and there it worked.

Here's an example of the accumulative view:

Hope it works for you.