ggplot2: multiple plots in a single row with a single legend

Question

I want a combined plot of two plots + their legend like this:

library(ggplot2) 
library(grid)
library(gridExtra)
dsamp <- diamonds[sample(nrow(diamonds), 1000), ]    
p1 <- qplot(price, carat, data=dsamp, colour=clarity)
p2 <- qplot(price, depth, data=dsamp, colour=clarity)
g <- ggplotGrob(p1 + theme(legend.position="bottom"))$grobs
legend <- g[[which(sapply(g, function(x) x$name) == "guide-box")]]
grid.arrange(arrangeGrob(p1+theme(legend.position="right"),p2+theme(legend.position="none"),legend,ncol=3,widths=c(3/7,3/7,1/7)))

However I do not want to guess the width of the plots and legends (and specify ncol) but have it extracted from p1 and p2 as shown here.

So I expect I would need something like this (adapted code from the link):

grid_arrange_shared_legend_row <- function(...) {
  plots <- list(...)
  g <- ggplotGrob(plots[[1]] + theme(legend.position="right"))$grobs
  legend <- g[[which(sapply(g, function(x) x$name) == "guide-box")]]
  lwidth <- sum(legend$width)
  grid.arrange(
    do.call(arrangeGrob, lapply(plots, function(x)
      x + theme(legend.position="none"))),
    legend,
    ncol = length(plots)+1,
    widths = unit.c(rep(unit(1, "npc") - lwidth, length(plots)), lwidth))
}
grid_arrange_shared_legend_row(p1, p2)

but this is not arranging the two plots in one row but rather one column:

This question is similar to this one here but different in that I am asking for the adapted widths as well. I am using code extracts both from that question + answer and the github.

Answer 1

Why don't you use facetting?

library(reshape2)
dmelt <- melt(dsamp, id.vars = c("price", "clarity"), measure.vars = c("carat", "depth"))
ggplot(dmelt, aes(x = price, y = value, color = clarity)) +
  geom_point() +
  facet_wrap(~ variable, scales = "free")