It is important to differentiate between a "correlation" and a cause and effect relationship. Just because your graphs may show an increase in P2P use at the same time as a decrease in CD sales does not mean the first "caused" the second; there is just a correlation.
There are other factors involved here including the resale of old music on CD. Sales of classic rock really fueled the CD boom. Eventually us old fogies bought all we wanted (I've got 4 versions of Pink Floyd's "Dark Side of the Moon"). Check this link for one person's opinion:
http://www.audiorevolution.com/news/...ssicrock.shtml
Sorry about the rant, but this concept is critical. And not just regarding this topic.
Rant over.
Good luck.