The percentage change on the portfolio history graph is actually not the profit!
The portfolio history graph is a graph of the 'total portfolio worth' and gives you the changes in your total portfolio value over time. Thus new purchases or fiat deposits also show as a change here, since they are changes in your portfolio's worth. The all-time value shown here is the increase in your portfolio worth since the beginning and not the profit.
For all-time profit, refer to the portfolio main screen.