Edit: I am more convinced than before that your trendline errs on the side of predicting a BTC/USD rate on the low side and can lead to a premature sell indicator.
The sell signal is when price = 3*trend
I understand this but if the trend is below where it should be then the sell signal will be premature.
the trend is never below where it should be, it is just a trend. The price can be below the trend.
Sell signal level has nothing to do with current price. It is predefined. It is only a function of time and trend (sell signal happens when current price = trend price * 3)
The trend is based on a model and rpietila's model has one major shortcoming. I came to this conclusion by analysing the early 2010 and 2009 data. The fact that there is something wrong here becomes clear when one looks at the graph and focuses on 2009 and early 2010. My first thought was that the data from this period was way off; however rpietila provided enough anecdotal evidence to convince me that his estimate was at least in the right ballpark. This led me to question the model instead. My conclusion is that using price for the log fit is incorrect and that market capitalization should be used instead. The impact of this is most profound in 2009. My preliminary results predict a trend-line right now of around 4000 per BTC rather than around 900 USD per BTC. It also predicted from the 2010 and later data a constant price around 0.005 USD per BTC for a significant part of 2009, vindicating rpietila's guess that I had severely questioned.