Weird. Do your numbers use a different date from the chart? At the top, in words, is
giving the number from 4 days ago and the percentage change from 4 weeks before that (popup gives the exact dates and numbers). In my case that's 8% -- and I can assure everyone it is not because my site is 8% more interesting than it was a month ago :)
But the graph goes 2 days beyond that, dropping to a count that is now 1 less than it was on the start date (same date for text and graph). My peak date was May 3d with a one-time spike of about 10% both before and after. No relation to the day I manually removed a cluster of long-outdated URLs; that was about a week later, with mysteriously no downward hiccup in the graph.
Wish there were a way to figure out if those are the same 10% of pages, indexed and then promptly de-indexed again, or just some unnerving coincidence.