I've been going on (and on) about the trend set in the first part of this year as being the true reference line: 1.8% daily growth.
I think taking it back to the underground days is flawed in that, only this year are we seeing anything like the real value. But if you see here (the last chart I made showing the entire Mt Gox history), you can see that trend albeit in a looser fashion, actually began in Feb 2012:
http://tonycoleby.com/shared/2013-04-11_bitcoin_growth.png
It is by far the longest vague trendline since July 2010. People, whether consciously or otherwise are looking at that line as a long-term reference.
If you are feeling super-bullish, you can extrapolate the trend from $47.40 on 17th March 2013 @ 1.8% daily growth and see that the line passes through $1,000 in about 6 months. What with the volatile nature of BTC, we could see that price in half the time if only temporarily.
I'm sure a lot of people have a price target in their head and $1,000 is a nice, round figure
ManBearPig,
A 1.8% per day increase seems about right.
In your graph, which is logarithmic, the gradual 30-40 degree ascending dotted line in the middle on the right parallels my understanding of BTC's future value increase. Your gradual line is ca 75 USD below my estimate, but both lines rise at about the same rate. 75 USD is not going to be so much difference when 1 BTC is over 1000 USD.