Transistor power dissipation rating
First, the power burnt up in a component is equal to the voltage drop of that component times the current going through it.  The transistor has a \$V_{CE}\$ of 3V when the collector current is 20A, so that works out to 60W.

Second, the power derating curve works by telling you the dissipation you can allow for a given case temperature.  You read across until you find your design case temperature, then up to find the allowable dissipation (or, usually, the other way -- in your case you'd start at 60W, then read across to find that you should design for a case temperature of around 135 degrees C).

Third, you'll need heat sinking to keep the case temperature down -- that thing won't dissipate 60W by itself.

Fourth (and beyond what you were asking), you need a base current of 200mA to maintain that 3V \$V_{CE}\$, and that's pushing into a base at a voltage of 4V.  You are not going to do that with a microprocessor GPIO pin.