Add "average line" to historical hashrate graphs

On the "my hashrate over the last X" graphs, add a horizontal line that is the average hashrate for the values during X. This would make it easy to visualize your actual hashrate average, taking into account any fluctuations (e.g. power outtages, dead cards, etc).

Alternatively, maybe just another stat in the left-hand stats column that is an "all-time hashrate average".