It's based on my last.fm plays (for each artist), but it's really not that accurate since 75% of logged plays have been from September 2013, I think, around when I actually started using last.fm (I imported about 9k plays and have almost 36k now). Which explains why even though I love Rihanna and have liked ha since ha debut single Pon De Replay, she has hardly any plays (since I haven't listened to her music much since early 2013).