Hi,
That's quite a project -- congratulations!
You could correct the graphs for each year for the number of heating degree days for that year compared to the average number of heating degree days.
I use this site for degree days:
http://www.degreedays.net/Not sure if they do outside the US, but I'd guess there is someplace you could get HDD for your area.
One potential source is to ask your local propane or oil delivery company, as they use HDDs to estimate when peoples tanks need to be filled.
I've found that this really tends to smooth out the year to year comparisons.
Gary