As you can see in 2010-2011 the then most accurate trendline (red line) seems quite robust but then, over a year, shifts to another exponent, the current one, which has been valid for a further year and a half starting in mid-2012.
Ln base (instead of log) makes it difficult to read. But the more dangerous design decision is to start the graph from Mt.Gox inception, which is really a totally arbitrary choice of a startdate. There has been Bitcoin trading before Mt.Gox, the fact that finding the data is hard does not give us the right to ignore it! I have estimated it to be a flat $0.005/1
BTC based on multiple isolated trades, and the ballpark is certainly correct because there has not been any trades below $0.001 or above $0.010 before the opening of Gox, which instantly lifted the price to a new level of about $0.05-$0.08. (Again, many have criticized this, but never given any recommendation about what might be better, NOR helped me to find more data on the trading in 2009-10.)
What kind of trading signals has that one given? Like
I told in its thread, mine has excelled in buyback zones - the previous 2 signals at $2.28 in October-2011 and $71 last summer were spot on, and this time the signal came at $460 some days ago.
There is data for early 2010 from New Liberty Standard and Bitcoin Market starting in January 2010.
http://newlibertystandard.wikifoundry.com/page/Exchange+Rate. There is also a reference from theymos indicating he felt New Liberty Standard was over charging, and that New Liberty Standard was the most visible exchanger at the time.
https://bitcointalksearch.org/topic/m.1143955. I have pointed out this data before. This data is based on 1 gram XAU via Pecunix. One can easily convert this to USD using the gold price at the time. There is little doubt in my mind that this model breaks down in 2009. I believe that an exponential model based on the market capitalization rather than price and the inclusion of the New Liberty Standard and Bitcoin Market data will address these shortcomings. The net effect of these shortcomings is to give premature
sell signals. A very good example is that sell signal given at the April 2011 low in the 0.6 USD range. If one takes a close look at the graph this sell signal is comparable in strength to the sell signal given for the April 2013 high.
http://bitcoincharts.com/charts/mtgoxUSD#tgSzm1g10zm2g25zvzl.
My conclusion is that one must treat a bitcoin
sell signal given by this model with extreme caution.
Edit: My arguments against rpietila's model should not be construed as making a case for the bears at this time in the market. This is a bull arguing that another bull is not bullish enough.