Trim Levels of Another Variable From Each Facet (Postprocessing split step)
trim_levels_in_facets(innervar)a function suitable for use in the pre
(list) argument of make_split_fun
character. The variable(s) to trim (remove unobserved levels) independently within each facet.
make_split_fun
Other make_custom_split:
add_combo_facet(),
drop_facet_levels(),
make_split_fun(),
make_split_result()