The graph showing 300 Kelvin is equal to 80 degrees F (27C) it goes to 440K (332F / 167C)
Since this is mostly above normal environmental conditions, one might assume it is intended to show how the device should be derated under extreme conditions.
Since the curve seems to be flattening out below 320K (116F / 47C), one might assume that the device performance is more linear at lower temperatures.