Can someone explain to me the Y-Axis here? I don't can't figure out the denominator of that percentage. I assume the numerator is "days destroyed". Is the denominator the total number coin "age days" at the start of a given day?
The percentage is (bitcoin_days_ever_destroyed / bitcoin_days_ever_created). Today there will be ~7.2 million bitcoin days created, because that is how many BTC are in circulation.