If pretty = 0 then the level names of a factor split attributes
are used unchanged. If pretty = NULL, the levels are presented
by a, b, … z, 0 … 5. If
pretty is a positive integer, abbreviate is
applied to the labels with that value for its argument
minlength.
If the lettering is vertical (par srt = 90) and adj is
not supplied it is adjusted appropriately.