I'd much prefer a line graph for avg mph history. It would display a vehicle's history a lot more clearly. Another solution could be to allow settings for "Recent Fuel-ups" to go back further and/or include the entire history.
If a line graph isn't possible, the bar graph should at least be optimized by making the bars thinner and pushing them closer together. There is a lot of visual space to fill.
(Love the site! Love it death now that there is an app!!)
The software we use for graphing isn't the most advanced in the world, so we're a bit limited in what we can do. I can see why a line graph would be good, but I think the bar graph works better for monthly totals—it's probably just a personal preference. As you add more months of fuel-ups the bars do get skinnier. Check out my prius to see an example. They are comically large if you just have three months worth of data, but they work.