Store output from gridExtra::grid.arrange into an object - r

Question

I am placing multiple plots into one image using gridExtra::grid.arrange and would like to have the option of saving the combined plot as an object that could be returned from within a function as part of a list of returned objects. Ideally, I would like to do this without printing the plot object.
The code below creates two plots, combines them with grid.arrange, and attempts to save the result into x. However, x evaluates to NULL and the plot is printed. The documentation for grid.arrange points me to arrangeGrob and suggests plotting can be turned off using plot=FALSE, but I get an error when I try that because FALSE is not a grob object.
Any suggestions for what I'm not understanding?
# R under development
# Windows 7 (32 bit)
# ggplot2 1.0.0
# gridExtra 0.9.1
p1 <- ggplot(mtcars, aes(x=factor(cyl), y=mpg)) + geom_boxplot()
p2 <- ggplot(mtcars, aes(x=factor(cyl), y=wt)) + geom_boxplot()
x <- gridExtra::grid.arrange(p1, p2)
x
Per the comments, I'm adding this edit. When I try it with arrangeGrob, I get no output at all.
> gridExtra::arrangeGrob(p1, p2)
> print(gridExtra::arrangeGrob(p1, p2))
Error: No layers in plot
> x <- gridExtra::arrangeGrob(p1, p2)
> x
Error: No layers in plot

Answer 1

The code in your edit does not work properly since you didn't load gridExtra.
library(gridExtra)
y <- arrangeGrob(p1, p2, ncol = 1)
class(y)
#[1] "gtable" "grob" "gDesc"
grid.draw(y)
Edit: since version 2.0.0, my comment about grid dependency below is no longer valid, since grid is now imported.
Edit: With gridExtra version >= 2.0.0, there is no need to attach either package,
p <- ggplot2::qplot(1,1)
x <- gridExtra::arrangeGrob(p, p)
grid::grid.draw(x)

Answer 2

Funny that this was asked so recently - I was running into this problem as well this week and was able to solve it in a bit of a hacky way, but I couldn't find any other solution I was happier with.
Problem 1: ggplotGrob is not found
I had to make sure ggplot2 is loaded. I don't completely understand what's happening (I admit I don't fully understand imports/depends/attaching/etc), but the following fixes that. I'd be open to feedback if this is very dangerous.
if (!"package:ggplot2" %in% search()) {
suppressPackageStartupMessages(attachNamespace("ggplot2"))
on.exit(detach("package:ggplot2"))
}
Somebody else linked to this blog post and I think that works as well, but from my (non-complete) understanding, this solution is less horrible. I think.
Problem 2: no layers in plot
As you discovered too, fixing that problem allows us to use grid.arrange, but that returns NULL and doesn't allow saving to an object. So I also wanted to use arrangeGrob but I also ran into the above error when gridExtra was not already loaded. Applying the fix from problem 1 again doesn't seem to work (maybe the package is getting de-attached too early?). BUT I noticed that calling grid::grid.draw on the result of arrangeGrob prints it fine without error. So I added a custom class to the output of arrangeGrob and added a generic print method that simply calls grid.draw
f <- function() {
plot <- gridExtra::arrangeGrob(...)
class(plot) <- c("ggExtraPlot", class(plot))
plot
}
print.ggExtraPlot <- function(x, ...) {
grid::grid.draw(x)
}
Hooray, now I can open a fresh R session with no packages explicitly loaded, and I can successfully call a function that creates a grob and print it later!
You can see the code in action in my package on GitHub.