Correlation is not causation. Maybe the quality of music has gone down, and people are no longer willing to pay $17 for a CD. Maybe if file sharing didn't exist, they still wouldnt' be buying it at that price. Maybe there's no place for that on your chart.
As an example, I don't share music (I have, but not in a long time--like the same time I was buying it), and I also haven't been buying it for a long time. I've purchased something on the order of 500 CDs, including maybe 30 in the last ten years.