Draw Legend Items With Different Colors in R

Use the
legend
Function to Add Legend to Plots in R 
Use Double
legend
Call to Draw Legend Items With Different Colors
This article will demonstrate how to draw legend items with different colors in R.
Use the legend
Function to Add Legend to Plots in R
The legend
function is used to draw legend objects on the plots. It takes x
and y
coordinates as the first two arguments to specify the legend’s position. Although, it can be specified using keywords like: "left"
, "bottomright"
, etc. The next parameter is called legend
, and it is represented as a vector of characters that need to be displayed in the legend. In this case, we draw lines and points denoted by a triangle or circle shape. Lines and points have different colors, and they need to be displayed in the legend accordingly. The following code snippet draws only shapes and text in the legend.
library(ggplot2)
plot( 0, type = "n", xlim = c(0,5), ylim = c(0,5) )
A < matrix( c( c(4,1,3,2), c(1,3,4,4)), ncol = 2 )
B < matrix( c( c(1,4,3,2), c(1,3,1,1)), ncol = 2 )
lines( A, col = "brown" )
points( A, col = "blue", pch = 17 )
lines( B, col = "cyan" )
points( B, col = "purple", pch = 19 )
legend( x = "topleft",
legend = c("Brown line, blue triangles","Cyan line, purple points"),
col = c("blue","purple"), lwd = 2, lty = c(0,0),
pch = c(17,19) )
We can also add lines by increasing the values of the lty
argument vector. Notice that ones correspond to the straight and uninterrupted line. On the other hand, pch
arguments represent the plotting symbols that map to 0:25
integer values and are described in points
function documentation.
library(ggplot2)
plot( 0, type = "n", xlim = c(0,5), ylim = c(0,5) )
A < matrix( c( c(4,1,3,2), c(1,3,4,4)), ncol = 2 )
B < matrix( c( c(1,4,3,2), c(1,3,1,1)), ncol = 2 )
lines( A, col = "brown" )
points( A, col = "blue", pch = 17 )
lines( B, col = "cyan" )
points( B, col = "purple", pch = 19 )
legend( x = "topleft",
legend = c("Brown line, blue triangles","Cyan line, purple points"),
col = c("brown","cyan"), lwd = 1, lty = c(1,1),
pch = c(17,19), merge = FALSE)
Use Double legend
Call to Draw Legend Items With Different Colors
The previous example displays the lines and shapes of a single item with the same color. This can be solved by the two invocations of the legend
function, each of which displays lines and shapes separately with different colors. Mind though, one of the calls should have empty vector values for the legend
parameter.
library(ggplot2)
plot( 0, type = "n", xlim = c(0,5), ylim = c(0,5) )
A < matrix( c( c(4,1,3,2), c(1,3,4,4)), ncol = 2 )
B < matrix( c( c(1,4,3,2), c(1,3,1,1)), ncol = 2 )
lines( A, col = "brown" )
points( A, col = "blue", pch = 17 )
lines( B, col = "cyan" )
points( B, col = "purple", pch = 19 )
legend( x = "topleft",
legend = c("Brown line, blue triangles","Cyan line, purple points"),
col = c("brown","cyan"), lwd = 1, lty = c(1,1),
pch = c(NA,NA) )
legend( x = "topleft",
legend = c("",""),
col = c("blue","purple"), lwd = 1, lty = c(0,0),
pch = c(17,19), bty= 'n')
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