It combines difficulty and bitcoin pricing over time.

Since MtGox pricing data doesn't exist before Sept 2010, I had to fake it with some rough data from bitmarket.
The data from the exchanges isn't in a clean one row per day format, so the graph will be a little bit off, but it shows the general trend well.

Hopefully, Sipa will add a chart like this to his site in the near future with code that can clean up the data a bit and combine pricing data from mtgox and bitmarket.

http://i.imgur.com/TU0fm.jpg