Are you running on the OE Michelin EnergySaver A/S tires or have you put winter tires on the Volt? Check the cold tire pressure. It should be at least 36 PSI. Recommend setting ~40 PSI cold pressure.
Like your gas changing to winter fuel mix? Most gas stations in the northern part of the US change over to winter blend in the Sept/Oct time frame, and back again in March/April. I saw a pronounced difference, even when running 91/93, in my old high-efficiency ICE before I got my Volt.Your graph shows that your efficiency changed and became worse in mid-September. Did anything change at that time