The graph is from this wiki page, which explains the axes slightly better: https://en.bitcoin.it/wiki/Bitcoin_Days_Destroyed#Graph_of_P... : the x-axis is time measured in blocks (created on average once every 10 minutes).
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.