Add a common Legend for combined ggplots

You may also use ggarrange from ggpubr package and set "common.legend = TRUE":

library(ggpubr)dsamp <- diamonds[sample(nrow(diamonds), 1000), ]p1 <- qplot(carat, price, data = dsamp, colour = clarity)p2 <- qplot(cut, price, data = dsamp, colour = clarity)p3 <- qplot(color, price, data = dsamp, colour = clarity)p4 <- qplot(depth, price, data = dsamp, colour = clarity) ggarrange(p1, p2, p3, p4, ncol=2, nrow=2, common.legend = TRUE, legend="bottom")

Update 2021-Mar

This answer has still some, but mostly historic, value. Over the years since this was posted better solutions have become available via packages. You should consider the newer answers posted below.

Update 2015-Feb

See Steven's answer below

df1 <- read.table(text="group   x     y   group1 -0.212201  0.358867group2 -0.279756 -0.126194group3  0.186860 -0.203273group4  0.417117 -0.002592group1 -0.212201  0.358867group2 -0.279756 -0.126194group3  0.186860 -0.203273group4  0.186860 -0.203273",header=TRUE)df2 <- read.table(text="group   x     y   group1  0.211826 -0.306214group2 -0.072626  0.104988group3 -0.072626  0.104988group4 -0.072626  0.104988group1  0.211826 -0.306214group2 -0.072626  0.104988group3 -0.072626  0.104988group4 -0.072626  0.104988",header=TRUE)library(ggplot2)library(gridExtra)p1 <- ggplot(df1, aes(x=x, y=y,colour=group)) + geom_point(position=position_jitter(w=0.04,h=0.02),size=1.8) + theme(legend.position="bottom")p2 <- ggplot(df2, aes(x=x, y=y,colour=group)) + geom_point(position=position_jitter(w=0.04,h=0.02),size=1.8)#extract legend#https://github.com/hadley/ggplot2/wiki/Share-a-legend-between-two-ggplot2-graphsg_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(p1)p3 <- grid.arrange(arrangeGrob(p1 + theme(legend.position="none"),                         p2 + theme(legend.position="none"),                         nrow=1),             mylegend, nrow=2,heights=c(10, 1))

Here is the resulting plot:

Roland's answer needs updating. See: https://github.com/hadley/ggplot2/wiki/Share-a-legend-between-two-ggplot2-graphs

This method has been updated for ggplot2 v1.0.0.

library(ggplot2)library(gridExtra)library(grid)grid_arrange_shared_legend <- function(...) {    plots <- list(...)    g <- ggplotGrob(plots[[1]] + theme(legend.position="bottom"))$grobs    legend <- g[[which(sapply(g, function(x) x$name) == "guide-box")]]    lheight <- sum(legend$height)    grid.arrange(        do.call(arrangeGrob, lapply(plots, function(x)            x + theme(legend.position="none"))),        legend,        ncol = 1,        heights = unit.c(unit(1, "npc") - lheight, lheight))}dsamp <- diamonds[sample(nrow(diamonds), 1000), ]p1 <- qplot(carat, price, data=dsamp, colour=clarity)p2 <- qplot(cut, price, data=dsamp, colour=clarity)p3 <- qplot(color, price, data=dsamp, colour=clarity)p4 <- qplot(depth, price, data=dsamp, colour=clarity)grid_arrange_shared_legend(p1, p2, p3, p4)

Note the lack of ggplot_gtable and ggplot_build. ggplotGrob is used instead. This example is a bit more convoluted than the above solution but it still solved it for me.