Add a legend to a base R chart

This page aims to explain how to add a legend to a plot made in base R. It is done using the legend() function. The main arguments are:

legend : names to display
bty: type of box around the legend. See graph #73
horiz : legend in column or in row
col : symbol color
pch: symbol type. See graph #6
pt.cex : symbol size
cex : text size
text.col: text color
topright : legend position: bottomright, bottom, bottomleft, left, topleft, top, topright, right, center.
inset : % (from 0 to 1) to draw the legend away from x and y axis
You can also give the X and Y coordinate of the legend: legend(3, 5, ...)

Note that an equivalent page exist concerning legends with ggplot2.

# Create data:
a=c(1:5)
b=c(5,3,4,5,5)
c=c(4,5,4,3,1)
 
# Make a basic graph
plot( b~a , type="b" , bty="l" , xlab="value of a" , ylab="value of b" , col=rgb(0.2,0.4,0.1,0.7) , lwd=3 , pch=17 , ylim=c(1,5) )
lines(c ~a , col=rgb(0.8,0.4,0.1,0.7) , lwd=3 , pch=19 , type="b" )
 
# Add a legend
legend("bottomleft", 
  legend = c("Group 1", "Group 2"), 
  col = c(rgb(0.2,0.4,0.1,0.7), 
  rgb(0.8,0.4,0.1,0.7)), 
  pch = c(17,19), 
  bty = "n", 
  pt.cex = 2, 
  cex = 1.2, 
  text.col = "black", 
  horiz = F , 
  inset = c(0.1, 0.1))

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