During rallies price does not increase as a pure exponential, like P(t) = A*B^(t-t0).  If it did, it would make a straight line on log-scale plots. 

The price does seem to grow exponentially during rallies, but starting on top of the previous base price: something like P(t) = A*B"(t-t0) + C.   The graph of that would still curve up like an exponential at first, even on a log-scale plot.  The log plot will begin to straighten out only after the first term gets much bigger than C.

In this case C is about $220, so the log-scale plot may straighten out only if and when the price hits $500 or so.