Used internally by the plotting functions to create faceting formulas.
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 in the horizontal space.
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 in the vertical space.
A formula which can be used in ggplot2::facet_grid() or NULL
if all arguments are NULL.