Marking all of your fuel-ups as "missed" is very unusual, so you probably ran into a bug doing that. You probably don't want to do that.
Marking a fuel-up as missed means that your odometer reading can't be read as the next one in a sequence. Looking at your fuel-up log it looks like you're trying to use tripometer tracking instead. All of your odometer readings are essentially the same—they should be getting higher each time.
You'll probably want to export everything you've entered so far, change the odometer column heading to km, and remove the missed_fuelup column altogether. Then create a new vehicle with tripometer tracking and do an import.
The reason that I've marked all me fuel-ups as missing is that I've been entering my fuel-ups in an excel-sheet before i discovered fuelly, so I have no record of the odometer/trip readings. I basically only have 2 correct odometer readings in this period.
What would you recommend me doing, so I can still use these data on fully, but without getting weird results?
Without a distance or odometer reading to use, your fuel-up data isn't going to be very useful here at Fuelly. I would start tracking with new fuel-ups today and maybe summarize your earlier spreadsheet data in a note. Entering false odometer readings where they're all pretty much the same reading isn't going to work well at all.
Yeah, I think it's useful for that. But it just won't be useful here at Fuelly. We have things set up so that an accurate distance or odometer reading is required for every entry.
For example, to calculate your average fuel economy we don't simply use an average of all of your fuel-up averages. We calculate the average using your total distance and total fuel used. So without a distance there's nothing to calculate.
I think maybe the confusion is around the idea of a fuel-up. The idea of a fuel-up here at Fuelly is that it corresponds with a trip to the fuel pump. So at Fuelly you enter your odometer reading at the time of the fuel-up and the amount of fuel you put in your tank. Then Fuelly calculates the distance you drove since your last fuel-up based on your previous odometer reading. It's not really going to work any other way.
So you can't, for example, enter an odometer reading and tell Fuelly you used 1,000 litres or gallons to get there. Each fuel-up here corresponds to an individual trip to a fuel pump. We do that because we calculate individual fuel-up fuel economy by using the difference from full. Here's more info about that: Why do I have to fill the tank all the way up every time I buy fuel?