Extract single plot from ggplot with facet_grid

I want to produce some plots using ggplot and facet_grid and save the plot as an object. My problem is that I also want to save each subgroup (i.e. each facet) as an object separately. My question is now if you can extract a single facet from facet_grid and save it as an object? Here is some simple code:

library(ggplot2)

ggplot(data = mtcars, aes(x = disp, y = mpg)) +
  geom_point() +
  facet_grid(. ~ am)

Now I'd like to produce two objects - one for am=0 and one for am=1.

Solution

I'm not sure why you wouldn't use subsetting, but you can extract individual facets from a facet grid.

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

p1 = ggplot(data = mtcars, aes(x = disp, y = mpg)) +
  geom_point() +
  facet_grid(. ~ am)


g1 = ggplotGrob(p1)


# Rows and columns can be dropped from the layout.

# To show the layout:
gtable_show_layout(g1)

# Which columns (and/or rows) to drop?
# In this case drop columns 5 and 6 to leave am = 0 plot
# Drop columns 4 and 5 to leave am = 1 plot

# am = 0 plot
g1_am0 = g1[,-c(5,6)]

grid.newpage()
grid.draw(g1_am0)


# am = 1 plot
g1_am1 = g1[,-c(4,5)]

grid.newpage()
grid.draw(g1_am1)