Add both margins to facet_grid but not the combination

Question

I want to show both the right and the bottom margin, but not the combination of both. Is this possible with facet_grid?

library(ggplot2)

ggplot(mtcars, aes(x = hp, group = vs)) + 
  geom_density() + 
  facet_grid(am ~ vs, switch = "y", margins = TRUE) 

I tried to fix it by manually providing the factors to the margin argument but that doesn't help. Also, the R Documentation does not help me.

Current outcome

Desired outcome

Answer 1

ggplot2 uses fixed idioms for naming key plot elements so we can use that deterministic condition to find the elements:

library(ggplot2)

ggplot(mtcars, aes(x = hp, group = vs)) + 
  geom_density() + 
  facet_grid(am ~ vs, switch = "y", margins = TRUE) -> gg

Build the plot into a gtable object:

gt <- ggplot_gtable(ggplot_build(gg))

Get the table cell names:

cells <- gt$layout$name

Figure out which one is the bottom corner panel and axis:

apply(
  apply(
    do.call(rbind, strsplit(grep("panel", cells, value=TRUE), "-"))[,2:3],
    2, as.integer), 
  2, max
) -> max_col_row

bottom_right_panel <- paste0(c("panel", max_col_row), collapse="-")
bottom_axis <- sprintf("axis-b-%s", max_col_row[2])

Figure out which ones are not the bottom corner panel:

paste0(
  grep(sprintf("^%s|%s$", bottom_corner, bottom_axis), cells, value=TRUE, invert = TRUE),
  collapse = "|"
) -> not_bottom_corner_panel

Only draw those:

grid::grid.draw(
  gtable::gtable_filter(gt, sprintf("^(%s)$", not_bottom_corner_panel))
)