You're a star, gbianchi!
Please fit us a function for the price dependent on time
P(t) = function (time-offset)
Thanks.
here the function you request:
P = e^(((days from 01/01/2012) * 0.0067) + 1.641))
So mas o menos, roughly speaking, approximately, rule of thumb:
Price = 6 * Exp (0.2 * months since 1/1/2012 )
Correcto?
really approximative ... but a sort of

Good. Thanks again!
Because with the new trick used above, I can make an even simpler function that also fits the price (of the past that is):
Price = exp ( 1/5 * months since 26.04.2011 )
For today, that is 0.04% different from your function, and even for 2020 that's only 7% off!
Edit: The "months" are taken to be "30 days".
If you want to know what happened
on the 26.4.2011, here is a search.
So here's the rule of thumb to remember:
The USD-price of bitcoin can be fitted with the function
Exp ( one fifth of the number of months since the 26.4.2011 )
(click)