force a regular plot object into a Grob for use in grid.arrange

Question

b <- ggplot(cars,aes(x=speed,y=dist))+geom_line()
grid.arrange(
    b,
    plot(cars),
    ncol=1
)

gives me the following error

Error in gList(list(grobs = list(list(x = 0.5, y = 0.5, width = 1, height = 1, : only 'grobs' allowed in "gList"

Let's assume my second graph has to come out of the plot function. How would one convert that output to a grob-like object so it plays nicely with grid.arrange ?

Answer 1

you can try with gridGraphics

library(gridGraphics)

grab_grob <- function(){
  grid.echo()
  grid.grab()
}

plot(cars)
g <- grab_grob()
b <- ggplot(cars,aes(x=speed,y=dist))+geom_line()
grid.arrange(
  b,g,
  ncol=1
)

or, alternatively, use gridBase.