Thanks for the explanation. The sum looks correct when selecting Oct 1 to Oct 30. This wasn't the behavior I was looking for but at least I know now the logic it's using.
Unfortunately this makes it less useful for the use cases I'm looking at. Ideally I would have liked the option (perhaps using a variation of the function offset) on a day of Dec 13 if I selected previous month the graph should display two line, one from Nov 1st to 30th, and the other going only from Dec 1st to the 13th. Same behavior with week and year offsets. This would be a lot more meaningful in terms of the comparison of numbers presented.