[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.