But the thicker ice gets, the better insulator it becomes: i.e., the less thermodynamic interaction there is between cold athmosphere above and liquid water below. That's why the growth slows down the thicker ice gets. The graph importance is _how_ - quantatively, - this process happens "with everything else being the same". I.e., if the water udner the ice suddenly changes to be much warmer than near 0C - then the graph is irrelevant; won't describe things happening. If the air above gets above 0C, or when polar night is not the case anymore - i.e., it's spring time, - the graph is also irrelevant.