The data seems a bit fishy to me. I have a hard time believing that gasoline consumption did not change for 22 years (1986 to 2008).

Thats not what the data represents. It represents sales by refiners to retail. That is buying gas from a Shell gas station (also a refiner) counts, buying gas from a mom + pop corner store who wholesale buys their gas from a refiner (or distributor) doesn't count. The graph seems to just say that some refiner sold off their retail gas stations (or something like that).