Bitcoin price history: growing by a factor of 3.2 per year

 The data is taken from blockchain.info, with the inital zeros stripped out. The y axis is the log of the dollar price (i.e. "1" means $10, "2" means $100, "3" means $1,000 and so on), so the best-fit straight line is actually an exponential growth curve. The slope (i.e. log of growth rate) is 0.5061, which translates to a growth rate of *3.2 (i.e. 320%) per year. Put another way, the long-term trend is to add another zero to the price every 2 years.This also shows that the surge over the last 6 months has merely taken the price back to the long term trend.