Hi, I number my tanks when I add them into fuelly. It *might* be a modest improvement to have numbers on the graph and against each tank.
For example, if you've entered 100 tanks for a particular vehicle, the list at the bottom would show tanks 91-100, and the graph would have the number 91-100 under/over each matching dot. Similarly, the best tank / last tank could have a number near the date.
I can see that numbers might be slightly easier to match up with the Log Book than dates when you want to compare the graph and the fuel-up details. But using numbers instead of dates would smooth out the graph and that was a big complaint about an older version of that graph.
I think it might work if we did more with individual fuel-ups and you had to refer back to individual fuel-ups more frequently. But as it is I think dates work better.