If the LabelUnits value of a Plot axis is non-zero, a unit description will be included in the descriptive label for that axis, otherwise it will be omitted. The default behaviour is to include a unit description unless the current Frame of the Plot is a SkyFrame representing equatorial, ecliptic, galactic or supergalactic coordinates, in which case it is omitted.
AST A Library for Handling World Coordinate Systems in Astronomy