It is a good idea. One issue with putting together this graph is the wide distribution of prices across the world. Fuelly doesn't yet have a good way to report location (beyond the location field in profiles, which only a handful of members have filled out), and without some breakdown by location average price doesn't mean much. Another issue is that Fuelly only currently supports US Dollars as a currency, and some users are entering prices in their home currency with no conversion, further obscuring the data. (We'll be testing currency conversion this week, so that will help once it's out there.)
So yes, great idea, we just need to jump some data hurdles before we get there.
As the site works now, no, we won't be adding a price graph for the reasons I mentioned above. You'll have to use a site like GasBuddy (USA/Canada) or PetrolPrices (UK) to track gas prices. We're going to keep Fuelly focused on personal fuel economy for the time being.