ggplot：构面之间的剪切线

Question

Say I have a plot like this:

# Load libraries
library(ggplot2)
library(grid)

# Load data
data(mtcars)

# Plot results
p <- ggplot(data = mtcars) 
p <- p + geom_bar(aes(cyl)) 
p <- p + coord_flip()
p <- p + facet_wrap(~am)

print(p)

Now, I want to plot lines all the way across both facets where the bars are. I add this:

p <- p + geom_vline(aes(xintercept = cyl))

which adds the lines, but they don't cross both facets. So, I try to turn off clipping using this solution:

# Turn off clipping
gt <- ggplot_gtable(ggplot_build(p))
gt$layout$clip[gt$layout$name == "panel"] <- "off"

# Plot results
grid.draw(gt)

but that doesn't solve the problem: the lines are still clipped. So, I wondered if this is specific to geom_vline and tried approaches with geom_abline and geom_line (the latter with values across ±Inf), but the results are the same. In other posts, the clipping solution seems to work for text and points, but presumably in this case the lines are only defined within the limits of the figure. (I even tried gt$layout$clip <- "off" to switch off all possible clipping, but that didn't solve the problem.) Is there a workaround?

Answer 1

library(grid)
library(gtable)

# Starting from your plot `p`
gb <- ggplot_build(p)
g <- ggplot_gtable(gb)

# Get position of y-axis tick marks
ys <- gb$layout$panel_ranges[[1]][["y.major"]]


# Add segments at these positions
# subset `ys` if you only want to add a few
# have a look at g$layout for relevant `l` and `r` positions
g <- gtable_add_grob(g, segmentsGrob(y0=ys, y1=ys, 
                                     gp=gpar(col="red", lty="dashed")), 
                     t = 7, l = 4, r=8)

grid.newpage()
grid.draw(g)

see ggplot, drawing multiple lines across facets for how to rescale values for more general plotting. ie

data2npc <- function(x, panel = 1L, axis = "x") {
  range <- pb$layout$panel_ranges[[panel]][[paste0(axis,".range")]]
  scales::rescale(c(range, x), c(0,1))[-c(1,2)]
}

start <- sapply(c(4,6,8), data2npc, panel=1, axis="y")

g <- gtable_add_grob(g, segmentsGrob(y0=start, y1=start),
                      t=7, r=4, l=8)