EDIT: or, of course, the line is drawn "by intuition" alone. nothing wrong with that, but would be good to know.
oda.krell, thanks for the feedback!
Yes it is simple linear regression (least squares fit). 1100 datapoints with slope projected 300 more. The data is presented raw (from bitcoincharts.com with no massaging).
I don't take this as gospel, just a rough guide. We are in the speculation forum and the best we can do in speculating is to make use of the data we have. There is no doubt that Bitcoin is on an exponential adoption phase, which has been seen with other technologies such as the take-up of mobile telephony. It will plateau at some point and skew level on charts such as the one I posted. We can have confidence that the regression line has short-term future relevance because of inertia within the Bitcoin ecosystem which is building all the time as more money and people get involved in many different countries.
"The fit curve is an exponential percentual to the chart least squares fit - in other words: I played around until it looked good."