If you take the averaging period down to 1 (box in the lower left), then the center line is the average temperature for each day. The top of the shaded region is the high for that day. If you zoom in on, say, 2008-05-15, you can see that the high was over 100 degrees that day.

In general, though, temperatures in SF rarely go above 80. When they do, it's newsworthy.