and the EU nation equivalents, along with low gas prices here are screwing up the year to year numbers. Personally, I hate that stat as it is about a "context free" statistic as any that is out there, and like Ian points out, really distorts the absolute value of the actual number.
We'd be better served by posting Ians graphs here that show not only year on year, but the trends that have been going on.
As it is, the year on year monthly number really begs for misinterpretation.
