Add a common Legend for combined ggplots

I have two ggplots which I align horizontally with grid.arrange. I have looked through a lot of forum posts, but everything I try seem to be commands that are now updated and named something else.

My data looks like this;

# Data plot 1                                   
        axis1     axis2   
group1 -0.212201  0.358867
group2 -0.279756 -0.126194
group3  0.186860 -0.203273
group4  0.417117 -0.002592
group1 -0.212201  0.358867
group2 -0.279756 -0.126194
group3  0.186860 -0.203273
group4  0.186860 -0.203273

# Data plot 2   
        axis1     axis2
group1  0.211826 -0.306214
group2 -0.072626  0.104988
group3 -0.072626  0.104988
group4 -0.072626  0.104988
group1  0.211826 -0.306214
group2 -0.072626  0.104988
group3 -0.072626  0.104988
group4 -0.072626  0.104988

#And I run this:
library(ggplot2)
library(gridExtra)


groups=c('group1','group2','group3','group4','group1','group2','group3','group4')

x1=data1[,1]
y1=data1[,2]

x2=data2[,1]
y2=data2[,2]

p1=ggplot(data1, aes(x=x1, y=y1,colour=groups)) + geom_point(position=position_jitter(w=0.04,h=0.02),size=1.8)

p2=ggplot(data2, aes(x=x2, y=y2,colour=groups)) + geom_point(position=position_jitter(w=0.04,h=0.02),size=1.8)

#Combine plots
p3=grid.arrange(
p1 + theme(legend.position="none"), p2+ theme(legend.position="none"), nrow=1, widths = unit(c(10.,10), "cm"), heights = unit(rep(8, 1), "cm")))

How would I extract the legend from any of these plots and add it to the bottom/centre of the combined plot?

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  0.358867
group2 -0.279756 -0.126194
group3  0.186860 -0.203273
group4  0.417117 -0.002592
group1 -0.212201  0.358867
group2 -0.279756 -0.126194
group3  0.186860 -0.203273
group4  0.186860 -0.203273",header=TRUE)

df2 <- read.table(text="group   x     y   
group1  0.211826 -0.306214
group2 -0.072626  0.104988
group3 -0.072626  0.104988
group4 -0.072626  0.104988
group1  0.211826 -0.306214
group2 -0.072626  0.104988
group3 -0.072626  0.104988
group4 -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-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(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.