All positions are rendered and it is up to the facet to decide which to use
render_strips(x = NULL, y = NULL, labeller, theme)
A data.frame with a column for each variable and a row for each combination to draw
A labeller function
A list with an "x" and a "y" element, each containing a "top" and "bottom" or "left" and "right" element respectively. These contains a list of rendered strips as gtables.