Add legends to plots in R software : the easiest way!


The goal of this article is to show you how to add legends to plots using R statistical software.

R legend function

To add legends to plots in R, the R legend() function can be used. A simplified format of the function is :

legend(x, y=NULL, legend, fill, col, bg)

  • x and y : the x and y co-ordinates to be used to position the legend
  • legend : the text of the legend
  • fill : colors to use for filling the boxes beside the legend text
  • col : colors of lines and points beside the legend text
  • bg : the background color for the legend box.


Example :

# Generate some data
x<-1:10; y1=x*x; y2=2*y1
plot(x, y1, type="b", pch=19, col="red", xlab="x", ylab="y")
# Add a line
lines(x, y2, pch=18, col="blue", type="b", lty=2)
# Add a legend
legend(1, 95, legend=c("Line 1", "Line 2"),
       col=c("red", "blue"), lty=1:2, cex=0.8)

R legend : tutorial on how add legends to plots using R software

To avoid repeating the above R code, we can create a custom plot function as follow :

makePlot<-function(){
  x<-1:10; y1=x*x; y2=2*y1
  plot(x, y1, type="b", pch=19, col="red", xlab="x", ylab="y")
  lines(x, y2, pch=18, col="blue", type="b", lty=2)
}

Title, text font and background color of the legend box

The arguments below can be used :

  • title: The title of the legend
  • text.font: an integer specifying the font style of the legend text; possible values are :
    • 1: normal
    • 2: bold
    • 3: italic
    • 4: bold and italic
  • bg: background color of the legend box
makePlot()
# Add a legend to the plot
legend(1, 95, legend=c("Line 1", "Line 2"),
       col=c("red", "blue"), lty=1:2, cex=0.8,
       title="Line types", text.font=4, bg='lightblue')

R legend : tutorial on how add legends to plots using R software

Border of the legend box

The arguments box.lty, box.lwd and box.col can be used to modify the line type, width and color for the legend box border, respectively.

# Remove legend border using box.lty = 0
makePlot()
legend(1, 95, legend=c("Line 1", "Line 2"),
       col=c("red", "blue"), lty=1:2, cex=0.8,
       box.lty=0)
# Change the border
makePlot()
legend(1, 95, legend=c("Line 1", "Line 2"),
       col=c("red", "blue"), lty=1:2, cex=0.8,
       box.lty=2, box.lwd=2, box.col="green")

R legend : tutorial on how add legends to plots using R softwareR legend : tutorial on how add legends to plots using R software

Specify legend position by keywords

The position of the legend can be specified also using the following keywords : "bottomright", "bottom", "bottomleft", "left", "topleft", "top", "topright", "right" and "center".

The effect of using each of these keywords are shown in the figure below :

R legend : tutorial on how add legends to plots using R software

Example 1: line plot

#  Example 1: line plot
makePlot()
legend("topleft", legend=c("Line 1", "Line 2"),
       col=c("red", "blue"), lty=1:2, cex=0.8)

R legend : tutorial on how add legends to plots using R software

Example 2: box plot

attach(mtcars)
boxplot(mpg~cyl, 
        xlab="Cylinders", ylab="Miles/(US) gallon", 
        col=topo.colors(3))
   
legend("bottomleft", inset=.02, title="Number of Cylinders",
   c("4","6","8"), fill=topo.colors(3), horiz=TRUE, cex=0.8)

R legend : tutorial on how add legends to plots using R software

Note that the argument fill indicates the colors to use for filling the boxes beside the legend text

Infos

This analysis has been performed using R statistical software (ver. 3.1.0).


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