B73567AMT, your graph shows both cabin and aircraft altitude against time. This is the usual way this subject is depicted and it causes a lot of misunderstanding. It is acceptable to depict the operation in terms of altitudes when considering normal operation but study of possible failure modes must be looked at from the perspective of absolute and differential pressures. Both cabin pressure and outside pressure decrease as the aircraft climbs, but cabin pressure is controlled to reduce at a slower rate than the rate of decrease in outside pressure, causing an increase in cabin differential pressure just as you say.
During descent the cabin differential pressure (the difference between cabin pressure and outside pressure) decreases again - so, just as subsidence assumed, the aircraft cabin pressure increases according to the schedule until it reaches the nominal ground level pressure at the selected landing altitude.
The key that subsidence is looking for is that the controller computes a suitable descent schedule based on the difference between outside pressure at the start of the descent mode and the assumed outside pressure at the selected landing elevation - the area shown in your graph as proportional control.
A complicating factor in the system shown in your graph is that during ground operation the cabin pressure is ramped up to be controlled at a pressure equivalent to 200 feet below runway elevation prior to take-off and ramped down from a pressure equivalent to 300 feet below runway elevation at landing. This is to prevent ear-popping changes of cabin pressure due to sudden operation of the outflow valve at lift-off and touch-down.