CODEWITHSUNDEEP
rggplot2
Bright
Bright

Reputation: 83

ggplot, drawing multiple lines across facets

I drew two panels in a column using ggplot2 facet, and would like to add two vertical lines across the panels at x = 4 and 8. The following is the code:

library(ggplot2)
library(gtable)
library(grid)

dat <- data.frame(x=rep(1:10,2),y=1:20+rnorm(20),z=c(rep("A",10),rep("B",10)))

P <- ggplot(dat,aes(x,y)) + geom_point() + facet_grid(z~.) + xlim(0,10)
Pb <- ggplot_build(P);Pg <- ggplot_gtable(Pb)

for (i in c(4,8)){
    Pg <- gtable_add_grob(Pg, moveToGrob(i/10,0),t=8,l=4)
    Pg <- gtable_add_grob(Pg, lineToGrob(i/10,1),t=6,l=4)
}

Pg$layout$clip <- "off"
grid.newpage()
grid.draw(Pg)

The above code is modified from:ggplot, drawing line between points across facets. And this is the output figure.

There are two problems in this figure. First, only one vertical line was shown. It seems that moveToGrob only worked once.. Second, the shown line is not exact at x = 4. I didn't find the Pb$panel$ranges variable, so is there a way that I can correct the range as well? Thanks a lot.

Upvotes: 8

Views: 3766

Answers (2)

baptiste
baptiste

Reputation: 77106

Updated to ggplot2 V3.0.0

In the simple scenario where panels have common axes and the lines extend across the full y range you can draw lines over the whole gtable cells, having found the correct npc coordinates conversion (cf previous post, updated because ggplot2 keeps changing),

library(ggplot2)
library(gtable)
library(grid)

dat <- data.frame(x=rep(1:10,2),y=1:20+rnorm(20),z=c(rep("A",10),rep("B",10)))

p <- ggplot(dat,aes(x,y)) + geom_point() + facet_grid(z~.) + xlim(0,10)
pb <- ggplot_build(p)
pg <- ggplot_gtable(pb)


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


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

pg <- gtable_add_grob(pg, segmentsGrob(x0=start, x1=start, y0=0, y1=1, gp=gpar(lty=2)), t=7, b=9, l=5)

grid.newpage()
grid.draw(pg)

enter image description here

Upvotes: 8

alistaire
alistaire

Reputation: 43344

You can just use geom_vline and avoid the grid mess altogether:

ggplot(dat, aes(x, y)) + 
    geom_point() + 
    geom_vline(xintercept = c(4, 8)) + 
    facet_grid(z ~ .) + 
    xlim(0, 10)

plot with vlines

Upvotes: 6

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