So if we call # times songs played per album / # songs on an album X . . .

You took the total songs played and divided by X? I think the 2nd graph that just shows X is the most accurate and if anything you should be taking X and dividing it by total.

Like if you were doing this with a statistic to look at which baseball team had the most hits by lefties, you'd just take the total hits by lefties per team and divide by # lefties on the team. To see the % of hits by lefties per team, then you'd take that number and divide hit by the total hits by lefties in the league, but not the other way around. Sorry to get pedantic, but I find this stuff more interesting to think about than my job!