Shared legend for a cowplot grid in R

In this article, we will discuss how to create a cowplot grid with the shared legend in the R Programming language.

To do this first we will make a basic cowplot grid with two plots having their own legends separately. To do so using the cowplot grid package in the R Language we use the plot_grid() function and pass all the plots that we need in the grid as parameters.

Syntax: plot_grid(plot1,plot2,label=<label-vector>, ncol, nrow)

Example:

A basic cowplot grid with two plots having separate legends.

R

# Create sample data
sample_data <- data.frame(
  name = c("Geek1","Geek2","Geek3",
           "Geek4","Geeek5") ,
  value=c(31,12,15,28,45))

# Load ggplot2 and cowplot
library("ggplot2")
library("cowplot")

# Create both plot and store in variable
plot1<-ggplot(sample_data, aes(x=name, y=value, fill=name)) +
        geom_bar(stat = "identity")

plot2<-ggplot(sample_data, aes(x="", y=value, fill=name))+
        geom_bar(width = 1, stat = "identity") +
        coord_polar("y", start=0)

# plot_grid() function adds both plots in a cowplot grid
plot_grid(plot1, plot2)

Output:

Shared Legend cowplot grid

To create a cowplot grid with shared legend there is no built-in method but the functionality can be achieved by following these steps:

Step 1: Create plots to be put in the grid without legend using:

plot + theme(legend.position = "none")

Step 2: Now combine both plots using the plot_grid() function and store that in a variable:

combined_plot<-plot_grid(plot1, plot2, ...... ncol)

Step 3: Now we extract the legend from one of the above plots to put it in the combined plot using:

legend <- get_legend( plot1 )

Step 4: Finally combine the combined plot with the derived legend using the plot_grid function to get the desired look for the shared legend plot.

plot_grid(combined_plot, legend, ncol)

Example:

Here, is an implementation of the above approach to get a cowplot grid with shared legend.

R

# Create sample data
sample_data <- data.frame(
  name = c("Geek1","Geek2","Geek3",
           "Geek4","Geeek5") ,
  value=c(31,12,15,28,45))

# Load ggplot2 and cowplot
library("ggplot2")
library("cowplot")

# Create both plot without legend and store in variable
plot1<-ggplot(sample_data, aes(x=name, y=value, fill=name)) +
        geom_bar(stat = "identity")+
        theme(legend.position = "none")

plot2<-ggplot(sample_data, aes(x="", y=value, fill=name))+
        geom_bar(width = 1, stat = "identity") +
        coord_polar("y", start=0)+
        theme(legend.position = "none")

# combine both plot using plot_grid()
combined_plot<-plot_grid(plot1, plot2,ncol=2)

# extract legend from plot1
legend <- get_legend(
  plot1 +
    guides(color = guide_legend(nrow = 1)) +
    theme(legend.position = "bottom")
)

# Combine combined plot and legend using plot_grid()
plot_grid(combined_plot, legend,ncol=1,rel_heights = c(1, .1))

Output: