r - Saving a graph with ggsave after using ggplot_build and ggplot_gtable

Question

I am modifying a graph built with ggplot by altering the data produced by ggplot_build (for a reason similar to Include space for missing factor level used in fill aesthetics in geom_boxplot). As far as I understand the help I found on this topic, I should be able to save the result by applying ggplot_gtable and arrangeGrob before calling ggsave on the results (Saving grid.arrange() plot to file).

However I obtain an error "plot should be a ggplot2 plot", also with this simple reproductible example:

require('ggplot2')
require('gridExtra')
df <- data.frame(f1=factor(rbinom(100, 1, 0.45), label=c("m","w")), 
                  f2=factor(rbinom(100, 1, 0.45), label=c("young","old")),
                  boxthis=rnorm(100))
g <- ggplot(aes(y = boxthis, x = f2, fill = f1), data = df) + geom_boxplot()
dd <- ggplot_build(g)

# Printing the graph works:
print(arrangeGrob(ggplot_gtable(dd)))

# Saving the graph doesn't:
ggsave('test.png',arrangeGrob(ggplot_gtable(dd)))

Can anybody explain why this does not work ? Is there a way to use ggsave after modifying the data by using ggplot_build() ?

(My version of the packages are gridExtra_0.9.1 and ggplot2_0.9.3.1)

Answer 1

it does not work because ggsave wants an object of class ggplot, while you're passing a grob. arrangeGrob will sometimes trick ggsave in pretending inheritance from ggplot, but only when at least one of the grobs belongs to this class; here, however, you're only passing a gtable.

Perhaps the easiest workaround is to clone ggsave and bypass the class check,

ggsave <- ggplot2::ggsave; body(ggsave) <- body(ggplot2::ggsave)[-2]

Edit: The dev version of ggplot2 no longer requires this hack*, as ggsave now works with any grob.

*PS: this hack works no longer, as arrangeGrob now returns a gtable, and its print method does not draw on a device.