The graph shows individual fuel-ups that have a calculated MPG. You have several fuel-ups marked partial. And if you take a look at the FAQ entry for partial fuel-ups you'll see that those fuel-ups do not have a calculated MPG, and the fuel-up immediately after does not have a calculated MPG. Fuelly needs a full tank to work from to calculate a MPG for an individual fuel-up.
Once you have a few full fuel-ups in a row the graph will update.
It is possible to do a partial fill if you just keep track of the gallons you have added and let the miles add up, don't report till you have made a complete fill up, and add the extra gallons to the fill up gallons. There are times say before payday where I can't fill up so I add three gallons or so and fill up after payday, look at my fill ups the most my tank will hold is 15 gallons when its really empty. Also I'll do that when I think the price per a gallon is a ripoff.
You could, but don't let the miles accumulate too far, or it will not report your mileage due to having too much data for your entry. Its best to just to do as few partials as possible. Always try to fill your tank each time. Its hard, but I still do a full tank on each fill up.