A helper function for building a ggplot facet. Used internally by the plotting functions.
facet_form(horiz = NULL, horiz2 = NULL, vert = NULL, vert2 = NULL)A character string denoting which column to use as
the first (horiz) and second (horiz2) level of faceting in
the horizontal direction. E.g. "M" or "species". A value of NULL (default)
indicates no faceting.
A character string denoting which column to use as
the first (vert) and second (vert2) level of faceting in
the vertical direction. E.g. "M" or "species". A value of NULL (default)
indicates no faceting.
A formula which can be used in facet_grid, or NULL if all
arguments are NULL