This vignette demonstrates how to make compound plots with a shared legend.

We begin with a row of three plots, without legend.

library(ggplot2)
library(cowplot)
library(rlang)

# down-sampled diamonds data set
dsamp <- diamonds[sample(nrow(diamonds), 1000), ]

# function to create plots
plot_diamonds <- function(xaes) {
  xaes <- enquo(xaes)
  ggplot(dsamp, aes(!!xaes, price, color = clarity)) +
    geom_point() +
    theme_half_open(12) +
    # we set the left and right margins to 0 to remove 
    # unnecessary spacing in the final plot arrangement.
    theme(plot.margin = margin(6, 0, 6, 0))
}

# make three plots
p1 <- plot_diamonds(carat)
p2 <- plot_diamonds(depth) + ylab(NULL)
p3 <- plot_diamonds(color) + ylab(NULL)

# arrange the three plots in a single row
prow <- plot_grid(
  p1 + theme(legend.position="none"),
  p2 + theme(legend.position="none"),
  p3 + theme(legend.position="none"),
  align = 'vh',
  labels = c("A", "B", "C"),
  hjust = -1,
  nrow = 1
)
prow

Now we add the legend back in manually. We can place the legend to the side of the plots.

# extract the legend from one of the plots
legend <- get_legend(
  # create some space to the left of the legend
  p1 + theme(legend.box.margin = margin(0, 0, 0, 12))
)

# add the legend to the row we made earlier. Give it one-third of 
# the width of one plot (via rel_widths).
plot_grid(prow, legend, rel_widths = c(3, .4))

Or we can place the legend at the bottom.

# extract a legend that is laid out horizontally
legend_b <- get_legend(
  p1 + 
    guides(color = guide_legend(nrow = 1)) +
    theme(legend.position = "bottom")
)

# add the legend underneath the row we made earlier. Give it 10%
# of the height of one plot (via rel_heights).
plot_grid(prow, legend_b, ncol = 1, rel_heights = c(1, .1))

Or we can place the legend between plots.

# arrange the three plots in a single row, leaving space between plot B and C
prow <- plot_grid(
  p1 + theme(legend.position="none"),
  p2 + theme(legend.position="none"),
  NULL,
  p3 + theme(legend.position="none"),
  align = 'vh',
  labels = c("A", "B", "", "C"),
  hjust = -1,
  nrow = 1,
  rel_widths = c(1, 1, .3, 1)
)

# now add in the legend
prow + draw_grob(legend, 2/3.3, 0, .3/3.3, 1)

One more example, now with a more complex plot arrangement.

# plot 1
p1 <- ggplot(iris, aes(Sepal.Length, Sepal.Width, color = Species)) + 
  geom_point() + 
  stat_smooth(method = "lm") +
  facet_grid(. ~ Species) +
  theme_half_open(12) +
  background_grid(major = 'y', minor = "none") + 
  panel_border() + 
  theme(legend.position = "none")

# plot 2
p2 <- ggplot(iris, aes(Sepal.Length, fill = Species)) +
  geom_density(alpha = .7) + 
  scale_y_continuous(expand = expansion(mult = c(0, 0.05))) +
  theme_half_open(12) +
  theme(legend.justification = "top")
p2a <- p2 + theme(legend.position = "none")

# plot 3
p3 <- ggplot(iris, aes(Sepal.Width, fill = Species)) +
  geom_density(alpha = .7) + 
  scale_y_continuous(expand = c(0, 0)) +
  theme_half_open(12) +
  theme(legend.position = "none")

# legend
legend <- get_legend(p2)

# align all plots vertically
plots <- align_plots(p1, p2a, p3, align = 'v', axis = 'l')
## `geom_smooth()` using formula 'y ~ x'
# put together the bottom row and then everything
bottom_row <- plot_grid(
  plots[[2]], plots[[3]], legend,
  labels = c("B", "C"),
  rel_widths = c(1, 1, .3),
  nrow = 1
)
plot_grid(plots[[1]], bottom_row, labels = c("A"), ncol = 1)