[hammock]

Yes but why doesn't it look like a pie slice - outlined by "clock hands." The circle graph treats the year like a clock. There is no reason that if Aug 5 is above 70 on average, that the coloring only goes halfway on that day. It's a binary yes/no.

[inglor]

I presumed that represents the number of days in that month where the temperature was above (or below) that certain threshold.

This is rather common in weather sites.

[hammock]

Ok so that's what I mean by missing variable - the "number of days in that month" is not represented on the graph.

It also makes more sense that this is plotting 12 points and smoothing the curve, in my mind me I had assumed it was a daily plot of the data.