Indeed, it's a good chart. I think a good reason for some of the movement is as you mentioned:

Also, it may have something to do with upcoming regulatory changes where US based users may have to disclose any transfers to self hosted wallet in excess of $3k.

So the left scale means that about 2000 Bitcoins were removed from the exchanges in the last year? If this is so, then relatively speaking this would not be sooo much... but it's definitely better than the other way around.