When you're partial pressure of Oxygen is low (0 on the X axis) (BAD), the cerebral vessels will dilate and allow for more blood flow (high on the Y axis). This is because, while brain pressure is a concern, oxygen perfusion to the brain is more important. As more blood flows up into the brain, the pressure in the brain increase. Inversely, as the partial pressure of 02 approaches normal oxygen levels, it tapers off and constricts a "normal" amount (50,50 on graph). Basically, as long as the p02 isn't below 50 the brain thinks its "Good enough". The normal arterial oxygen is 100, so thats actually a pretty huge drop its willing to tolerate.
Similarly, when pCO2 is high (X=100)(BAD), the cerebral vessels will also dilate to get rid the CO2 waste asap (high on the Y Axis). Same reason as above, maintaining O2/CO2 levels is more important than worrying about brain pressure. Inversely, as the dangerous pCO2 drops, the vessels constrict, satisfied with the gas getting to/away from the brain. The primary difference is that there is no "good enough" for pCO2. The lower the CO2 gets, the more they will constrict in satisfaction. The "trick" associated with this is that you can manually hyperventilate, pushing pCO2 way below normal (40) to say 30, forcibly constricting the vessels way more than usual. If you look on the graph, This will reduce cerebral bloodflow (30,30). Thus, it will reduce intracranial pressure if there happens to be an alternative source of pressure you want to mitigate (tumor, swelling, etc).
The MAP is just demonstrating that a higher arterial pressure will push more blood into the brain. This is because the arteries in other areas of the body are constricted(or youre hypervolumic or whatever), and there is more resistance in other directions.
