3 years of gas consumption logging

For the last 3 years, since we have been in our current house, I have been taking very regular readings of the gas and electricity meter, usually daily, sometimes, skipping a day or 2. Recently, I have automated that process with the help of an Arduino and some additional electronics. That has removed the burden of collecting the data, and allows to see the evolution of the consumption to the minute! (note to self, explain the circuit to collect gas data!)
But all this accumulated data also has a good purpose, besides making nice graphics. It allows me to evaluate the measures I put in place to reduce the overall energy consumption.
In the first year, we only had an old Honeywell thermostat, manually operated. That was replaced in the second year by a fully automated system (see post regarding the uptime of the server). In the 3rd year nothing changed, but now that the 4th year is about to start, I am about to finish the roof insulation. All the accumulated data allows me to compare the increase in performance.
So here is a graphic of the monthly gas consumption for the last 3 years, along with the average temperature for that month as reported by the KNMI (dutch meteorologic institute) for Holland.

It is funny to see the relation to the temperature. In fact, from the data of the first year, I have managed to calculate a second order curve which relates temperature to gas consumption, so for a given monthly average, I can calculate how much the consumption would be. It is using this curve that I calculate my savings. I'll elaborate more on that soon.

Worth noticing is also the fact that December 2008 was much colder than December 2007, yet the gas consumption remained similar. I attribute that to the roof insulation, which I did mostly during November (not yet 100% finished).