Save plot and legend to separate files?

Question

I have some datasets that contain more legend entries than can comfortably be distinguished with colors or at all displayed with symbols. It's effectively a rainbow, but across so many legend entries that they make the plots much higher than wide.

Since the legends are not really as important as comfortable sizing of the plot, I merely preview and remove them before saving the plots to PNGs.

Like this:

library(ggplot2)
p <- ggplot(diamonds, aes(cut, depth)) + geom_point(aes(colour = factor(carat), size = price))
p
p <- p + theme(legend.position = "none")
p

However, having only the choice of either skewing the plot height or kicking the legend out completely, is a bit frustrating. A neat compromise would be to have the legend in a separate PNG, so it can be checked when really necessary. Is there a way to do this?

Answer 1

library(ggplot2)
p <- ggplot(diamonds, aes(cut, depth)) + geom_point(aes(colour = factor(carat), size = price))

#extract legend
#https://github.com/hadley/ggplot2/wiki/Share-a-legend-between-two-ggplot2-graphs
g_legend <- function(a.gplot){
  tmp <- ggplot_gtable(ggplot_build(a.gplot))
  leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
  legend <- tmp$grobs[[leg]]
  return(legend)
}

mylegend <- g_legend(p)
library(grid)
grid.draw(mylegend)

Just plot to different devices.

Answer 2

This is possible with cowplot and ggpubr.

library(cowplot)
my_legend <- get_legend(your_ggplot_object)
library(ggpubr)
as_ggplot(my_legend)