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This is a cumulative graph of the outstanding orders on the MtGox exchange. It shows how much BTC/USD will have to be bought/sold before the market rate hits a certain USD price. For example, let's say you want to know how much USD would have to be spent buying BTC before the price reaches 40 USD. First find 40.0 on the bottom axis. Then find where the blue line intersects $40.0. Now find what value that corresponds to on the right axis. If it corresponds to $200000 then that's how much USD would have to be spent buying BTC before the price reached 40 USD.