I don't believe you have this feature but it would be really useful to be able to make comparisons graphically on an annual basis i.e. the x-axis would be Jan-Dec and a different coloured line would show the trend for each year. This would allow one to compare seasonal trends and easily spot issues based on what mpg you expect at a particular time of year.
Another option would be to click the "Chart all Fuel-ups" link under your Recent Fuel-ups chart. You should be able see pretty quickly how this August compares with last August. It's not as fancy as having two trend lines together on the same graph, but it should give you the same data at a glance.