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.