When looking at a perfherder graph for my push, it shows me the last 14 days of a repo. But when doing pushes for which I want to use perfherder, I will typically stick to the same base revision for a long time rather than rebasing (because I did a try push with --rebuild LARGENUMBER to compare against, and don't want to invalidate my baseline.) That means that over time, the "last 14 days" becomes less and less relevant.
It would be nicer if it did 14 days around the date of the revision I'm based on. (Like, 7 days before and 7 days after, or 10 days before and 4 days after if my base was only from 4 days ago.)