How about a running average on the chart?
Would it be possible to put a running average curve in the graph? Depending on the day, the gas station, and just how much I have to fill up, my car can take anywhere from 7-10 gallons on a fillup from approximately the same spot on the fuel gage. If I fill up a little short one week, it'll obviously need more gas the next week, so things average out in the near-term. If you just had an average at each fillup of that fillup, the one before, and the one after, with the current fillup bing just an average of the current and last fillups, it would be just about perfect.
(I'm thinking it ought to be a spline curve.)