splitting or removing graphs after arrangeGrob

Question

I created a graph with ggplot and later I used arrangeGrob to combine those graphs. Is there a way to remove parts a graph from this combined plot? Or maybe extract?

Here is a minimal example:

library(ggplot2)
library(gridExtra)
df <- data.frame(x=rnorm(20), y=rnorm(20), y2=rnorm(20))
g1 <- ggplot(df, aes(x, y)) + geom_point()
g2 <- ggplot(df, aes(x, y2)) + geom_point()
g <- arrangeGrob(g1,g2, widths=c(3.5,7.5), ncol=2)
print(g)

I would like to remove one of the two plots.

Answer 1

First, use grid.ls() to see a listing of the grobs that make up the plot. Here, you'll be looking for the names of the two gTree objects that encode the individual plots. (Compared to lattice , ggplot2 's naming of component grobs is relatively unhelpful, although in this case, it's not too hard to see which pieces you'll want to extract.)

grid.ls()
# GRID.arrange.90
#   GRID.frame.84
#     GRID.cellGrob.85
#       GRID.frame.5
#         GRID.cellGrob.44
#           GRID.gTree.42
#             GRID.clip.43
#             layout
#         GRID.cellGrob.83
#           GRID.gTree.81
#             GRID.clip.82
#             layout
#     GRID.cellGrob.86
#       GRID.null.1
#     GRID.cellGrob.87
#       GRID.null.2
#     GRID.cellGrob.88
#       GRID.null.4
#     GRID.cellGrob.89
#       GRID.null.3

Then, you can extract and plot them like this:

gg1 <- getGrob(g, ("GRID.gTree.42"))
grid.draw(gg1)

gg2 <- getGrob(g, ("GRID.gTree.81"))
grid.draw(gg2)

Answer 2

grid graphics are complicated nested trees of things. A bit (okay, a lot) of trial and error managed to get your two plots out like this:

 > plot(g$children[[1]]$children[[1]]$children[[1]]$children[[1]]$children[[1]]$children[[2]])
 > plot(g$children[[1]]$children[[1]]$children[[1]]$children[[2]]$children[[1]]$children[[2]])

There's probably an easier way...

Answer 3

If you don't have to use arrangeGrob : It is possible to extract grobs from a gtable layout. It takes longer to set up the layout, but the extracting of the required element is straightforward.

library(gtable)
library(grid)
gt = gtable(unit(c(3.5, 7.5), "null"), unit(1, "null"))
gt = gtable_add_grob(gt, ggplotGrob(g1), l = 1, t = 1)
gt = gtable_add_grob(gt, ggplotGrob(g2), l = 2, t = 1)

plot(gt)
plot(gt[, 1])
plot(gt[, 2])

If you want to keep the size and positioning of the extracted plot the same as its size and positioning in the combined plot:

EDIT: Using Baptiste's suggestion:

gt = gtable(unit(c(3.5, 7.5), "null"), unit(1, "null"))
gt = gtable_add_grob(gt, ggplotGrob(g1), name = "g1", l = 1, t = 1)
gt = gtable_add_grob(gt, ggplotGrob(g2), name = "g2", l = 2, t = 1)

grid.newpage()
grid.draw(gtable_filter(gt, "g2", trim = FALSE))

grid.newpage()
grid.draw(gtable_filter(gt, "g1", trim = FALSE))

Original:

# Keep g2
p2 = gt
p2$layout = gt$layout[-1, ]
p2$grobs = gt$grobs[-1]
grid.newpage()
grid.draw(p2)

# Keep g1
p1 = gt
p1$layout = gt$layout[-2, ]
p1$grobs = gt$grobs[-2]
grid.newpage()
grid.draw(p1)