See the attached. This is a basic census problem, where you want to have a row for each transition (arrival/departure), and then you can use a running sum. I set that up using Custom SQL to create a union query in. There is still one issue to be dealt with, though, and that's when there's not at least one row for each month. You can see this in the "not padded line" worksheet in the attached, where there's no data for Feb 1 and the line connects Jan 1 to Mar 1. So it looks like there are 5.5 employees in February, when it should be 5. To get around that, I added a table of months and union'ed that in the Padded Custom SQL data source, and created views that work.
Nice job Jonathan. I see a utopia where perhaps we don't have to do any padding....
Thanks, and nice to see you active again on the forums!
I like your utopia, mine would have that too, along with a natural language interface to addressing and partitioning.