Great idea, and something we'd definitely like to add to the site. We're planning a Reports feature that lets you assemble reports and graphs based on time periods you specify. Unfortunately, Fuelly is a side project for us, and it's hard to find time to add features to the site.
For now, you might look at the
export feature and create your own custom reports yourself in a spreadsheet program. Once all of your fuel-ups are in Excel, it's fairly easy to slice and dice them in various ways. Of course this requires familiarity with Excel, so it's not ideal.
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