The Presidents (way) under the line might be regarded as overperformers. They didn’t have much experience going into office, but did pretty well once there. The ones (way) above the line had lots of experience but were not good Presidents.
The two Bushes and Clinton are not shown since it is much too early to draw historical judgments on them. One needs a very vivid imagination to perceive the data points falling along the blue line. There does not appear to be any relation between experience and greatness.