The above graph was created by BTC Design and shows what your return on investment would have been based on purchase date. The top part of the graph shows three lines, one for what your return would be if you sold your Bitcoin one year after purchase, two years after purchase, or if you waited until last month to sell.
The bottom part of the graph is a logarithmic chart of Bitcoin price over time. Logarithmic charts show relative movement rather than absolute movement. For example, in a logarithmic chart, a price increase from $.50 to $1.00 is shown the same as an increase from $1.00 to $2.00, but in a linear chart, the increase from $1.00 to $2.00 is shown as much larger.