This timeline maps the listening trends of last.fm users in New York city and compares those trends to what the world has been listening to. The graphic does not indicate what the unit of measure is which determines the size of each artist's progression, however I would assume that the height relates to the number of plays or the number of listeners who frequent each artist.
I would be interested to see a comparison between a U.S. city and the entire U.S. or between two U.S. cities. Comparing one U.S. city to the entire world doesn't seem like an appropriate comparison. There is no way to sample the entire world since many places do not have internet access let alone access to digital music. A comparison between two metropolitan areas would give a more accurate sample for a comparison of this type.
The one thing I do love about this graphic is its clarity in displaying the differences in what could be considered mainstream music.