How to plot just the legends in ggplot2?

Question

I'm currently working with igraph and have colour labelled my vertices. I would like to add a legend Indicating what each colour represents.

What I can think of at this point is to use ggplot2 to print only the legend and hide a bar plot. Is there a way to just output the legend?

Answer 1

Here are 2 approaches:

Set Up Plot

library(ggplot2) 
library(grid)
library(gridExtra) 

my_hist <- ggplot(diamonds, aes(clarity, fill = cut)) + 
    geom_bar() 

Cowplot approach

# Using the cowplot package
legend <- cowplot::get_legend(my_hist)

grid.newpage()
grid.draw(legend)

Home grown approach

Shamelessly stolen from: Inserting a table under the legend in a ggplot2 histogram

## Function to extract legend
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]] 
    legend
} 

legend <- g_legend(my_hist) 

grid.newpage()
grid.draw(legend) 

Created on 2018-05-31 by the reprex package (v0.2.0).