I can advice the I do not see any picture of graph uploaded as per your mentions. However, from the information you mentioned, the data instability you experience might be due to your vehicle having 2 smaller fuel tanks instead of one. If this is the case, then it would not be possible to acquire accurate data because our device will detect input from both tanks. What makes the voltage jump up and down is due to each tank having its own fuel pump which sends different information to the dashboard where that information is being processed while our devices take the information from the first point of contact where the readings are not processed. This could be a potential issue why you might not receive accurate information.
If you have singular tank, then it might be because you did not calibrate your device accurately, please see our Wiki page here for more information on how to perform calibration calculations.