Curious about the distribution at the bottom (mainstreamness increases as you go right, opposite of the typical long-tail distribution charts), I found the Last.fm stats blog where Vince explains their definition of mainstream and the histogram.
Until now – to be 100% mainstream – a band had to have as much listeners as the 10 bands with the most listeners have in average. Sometimes this created strange results as some bands would always have more listeners….now I changed it to be “average listeners of top 5 bands in last.fm”. This way there aren’t anymore users with more than 100% mainstreamity.
People on the stats blog suggest that Vince should shift the measure so that the peak/mode (they say median) is 50% mainstream. I say don’t arbitrarily shift anything and see if the CBS deal increases the number of “mainstream” listeners or otherwise moves the distribution around. Or better yet they should make the mainstream axis logarithmic scale – band popularity should show an exponential effect, so you have to tease out the small discrepancies between people listening to little-known bands.