One shared legend for a cowplot grid in R

There is a vignette that shows how to do this.

The approach is to build your plots with the legend hidden theme(legend.position="none"). Then extract the legend grob from one of those objects.

grobs <- ggplotGrob(pfour)$grobs
legend <- grobs[[which(sapply(grobs, function(x) x$name) == "guide-box")]]

Then plot the legend as a seperate 'plot'. To have the legend at to the right you might do:

# build grid without legends
pgrid <- plot_grid(pone, ptwo, pthree, pfour, ncol = 2)
# add legend
p <- plot_grid(pgrid, legend, ncol = 2, rel_widths = c(1, .1))

You can use ggarrange function from ggpubr package. It has a logical argument common.legend. You just need to set it TRUE. In your case a code chunk will be:

library(ggpubr)

ggarrange(ponezoom, ptwozoom, pthreezoom, pfourzoom,
align='h', labels=c('A', 'B','C','D'),
common.legend = T)

See example with mtcars dataset:

library(tidyverse)
library(ggpubr)

# Create first plot
mtcars %>% 
  ggplot(aes(mpg, hp, color = factor(cyl))) +
  geom_point(size = 2) +
  theme_minimal()-> plot1

# Create second plot
mtcars %>% 
  ggplot(aes(disp, drat, color = factor(cyl))) +
  geom_point(size = 2) +
  theme_minimal() -> plot2

# Create grid
ggpubr::ggarrange(plot1, plot2, # list of plots
                  labels = "AUTO", # labels
                  common.legend = T, # COMMON LEGEND
                  legend = "bottom", # legend position
                  align = "hv", # Align them both, horizontal and vertical
                  nrow = 2)  # number of rows

Voila: