Am I the only one who can't read this graph? It looks like it's rotated 90 degrees and there's no time axis so I don't see what this is representing. I don't understand what the legend for the Y axis means.
It's a market depth chart - the orange line on the left represents the cumulative standing buy orders, the blue line represents the standing sell orders, the green line is the historical price per trade with the latest trade (thus value) being at the bottom. The Y axis is a representation of the bitcoin value, and the X axis is the representation of the USD buy/sell prices.
Here's a traditional market chart showing USD vs BTC since the beginning of Bitcoin. Salient features: enormous bubble a year ago, and many months of stability at $5 before the recent runup.
http://bitcoincharts.com/charts/mtgoxUSD#tgSzm1g10zm2g25zv