If there are no level changes in the tanks over a long period of time that would be detected by the fuel sensor voltage, it is possible that the number of points in the fuel graph will be lower.
This does not mean that the sensor is not working. When you start driving, changes will start to be detected more often, and the number of points on the graph will increase accordingly.
📘 If this happens with a vehicle that is driving, there is a possibility that the fuel sensor is not working correctly, or the additional tank is disconnected from the fuel flow to the main tank.