In some cases the fuel consumption is not shown properly, the graph contains gaps...why?

The fuel consumption is calculated from the mileage difference between current fill-up point (N) and the last fill-up point (N-1), as well as the amount of fuel filled in at the current point (N). This works fine as long as the vehicle is always filled up. If there are phases where the vehicle was not filled up (checkbox not marked during entry), the software automatically uses the latest point where the vehicle was filled up for the determination of fuel consumption (otherwise there would be no mathematical basis to calculate).
The amounts filled in during the phases where the vehicle was not filled up are accumulated for this calculation.