The tank is cone shaped.
For the first part of the cone, for every increase in measurement on the dipstick, the volume of liquid is greater. So the first part of the graph would be accelerating.
As the dipstick goes down further, the volume of liquid would be fairly constant relative to the measurement of the dipstick. (straight line).
However, at the final part of the cone, an increase in measurement on the dipstick would result in a lower increase in volume. (decelerating part of the graph).