This is definitely something we want to do. We've been picturing a "reports" feature that lets you get charts and data from your fuel-ups in a number of different ways including by date. It's still on the drawing board so it's a ways into the future.
I like to see slices of my fuel-up history too, and I've been using tags for that. For example, I just took a road trip from Oregon to California, and I tagged all fuel-ups for that trip "catrip". Then I can view all of those fuel-ups in a single page here. I don't get any nice charts, but I can quickly get a sense if those fuel-ups are outside the norm. (They're a little better than average for me.) So that might be one thing to try in the meantime.
When fuel is entered check that the miles per gallon calculated falls within a standard deviation of the average MPG previously calculated if there are more than 5 fuel fill-ups entered. If outside this range then exclude this fill-up from the average calculation but have link appear on the home page that optionally allows to see this non-matching data and possibly include it in case it really belongs (maybe a hybrid?)
Thanks for the idea ChefTom. We're discussing this a bit over here. Basically, we don't want to get too fancy and frustrate people as they're entering data. But please add any further comments to other thread.