When I'm using to definition special weather conditions to create a monthly overview of temperatures each month has one day less, according to the attached graph December 2009 has 30 days, January 2010 has 30 days and February has 27 days. Time bucket is 1 month.
Only difference I can see is the order of entries. Could you please try with high-to-low ordering? This also has the benefit that colors are applied more as expected (blue=cold, red=hot).
If this solves the issue I have a hint what kind of bug I have to search for.
When I'm using this definition, this is the result.
To count the days I only can use the last three months, those are complete, in November 2009 I've installed a battery backup to cover the lots of short power down periods.