Is it possible to draw the axis line first, before the data?

Question

This is a follow up to my previous question where I was looking for a solution to get the axis drawn first, then the data. The answer works for that specific question and example, but it opened a more general question how to change the plotting order of the underlying grobs. First the axis, then the data.

Very much in the way that the panel grid grob can be drawn on top or not.

Panel grid and axis grobs are apparently generated differently - axes more as guide objects rather than "simple" grobs. (Axes are drawn with ggplot2:::draw_axis(), whereas the panel grid is built as part of the ggplot2:::Layout object).

I guess this is why axes are drawn on top, and I wondered if the drawing order can be changed.

# An example to play with 

library(ggplot2)
df <- data.frame(var = "", val = 0)

ggplot(df) + 
  geom_point(aes(val, var), color = "red", size = 10) +
  scale_x_continuous(
    expand = c(0, 0),
    limits = c(0,1)
  ) +
  coord_cartesian(clip = "off") +
  theme_classic() 

Answer 1

A ggplot can be represented by its gtable. The position of the grobs are given by the layout element, and "the z-column is used to define the drawing order of the grobs".

The z value for the panel, which contains the points grob, can then be increased so that it is drawn last.

So if p is your plot then

g <- ggplotGrob(p) ;
g$layout[g$layout$name == "panel", "z"] <-  max(g$layout$z) + 1L
grid::grid.draw(g)

However, as noted in the comment this changes how the axis look, which perhaps, is due to the panel being drawn over some of the axis.

But in new exciting news from dww

if we add theme(panel.background = element_rect(fill = NA)) to the plot, the axes are no longer partially obscured. This both proves that this is the cause of the thinner axis lines, and also provides a reasonable workaround, provided you don't need a colored panel background.