Since the speed of the alternator would be dependent on the size of the crank pulley, and they don’t know that when testing an alternator by itself, I would assume that the rpm is the speed of the alternator.
The alternator is making some current anytime it is rotating, the graph is just the range it was tested in.
If you want to know how much current it is making at engine speed, you can figure it out as follows.
If you have a 2 1/2 inch alternator pulley and a 7 inch crank pulley (like mine), the alternator would be spinning 7/2.5 = 2.8 ( the ratio of the pulleys) so the alternator spins about 2.8 times faster than the engine. So if you divided the rpm numbers on the chart by about 3, you would be close. You can measure your pulleys and get an exact answer for your engine.
