Data

ToothGrowth data is used in the examples below :

# Convert the variable dose from numeric to factor variable
ToothGrowth$dose <- as.factor(ToothGrowth$dose)
head(ToothGrowth)

##    len supp dose
## 1  4.2   VC  0.5
## 2 11.5   VC  0.5
## 3  7.3   VC  0.5
## 4  5.8   VC  0.5
## 5  6.4   VC  0.5
## 6 10.0   VC  0.5

Make sure that the variable dose is converted as a factor variable using the above R script.

Example of plot

library(ggplot2)
p <- ggplot(ToothGrowth, aes(x=dose, y=len, fill=dose)) + 
  geom_boxplot()
p

Change the legend position

The position of the legend can be changed using the function theme() as follow :

p + theme(legend.position="top")
p + theme(legend.position="bottom")

The allowed values for the arguments legend.position are : “left”,“top”, “right”, “bottom”.

Note that, the argument legend.position can be also a numeric vector c(x,y). In this case it is possible to position the legend inside the plotting area. x and y are the coordinates of the legend box. Their values should be between 0 and 1. c(0,0) corresponds to the “bottom left” and c(1,1) corresponds to the “top right” position.

p + theme(legend.position = c(0.8, 0.2))

Change the legend title and text font styles

# legend title
p + theme(legend.title = element_text(colour="blue", size=10, 
                                      face="bold"))
# legend labels
p + theme(legend.text = element_text(colour="blue", size=10, 
                                     face="bold"))

Change the background color of the legend box

# legend box background color
p + theme(legend.background = element_rect(fill="lightblue", 
                                  size=0.5, linetype="solid"))
p + theme(legend.background = element_rect(fill="lightblue",
                                  size=0.5, linetype="solid", 
                                  colour ="darkblue"))

Change the order of legend items

To change the order of items to “2”, “0.5”, “1” :

p + scale_x_discrete(limits=c("2", "0.5", "1"))

Remove the plot legend

# Remove only the legend title
p + theme(legend.title = element_blank())
# Remove the plot legend
p + theme(legend.position='none')

Remove slashes in the legend of a bar plot

# Default plot
ggplot(data=ToothGrowth, aes(x=dose, fill=dose)) + geom_bar()
# Change bar plot border color, 
# but slashes are added in the legend
ggplot(data=ToothGrowth, aes(x=dose, fill=dose)) +
  geom_bar(colour="black")
# Hide the slashes: 
  #1. plot the bars with no border color,
  #2. plot the bars again with border color, but with a blank legend.
ggplot(data=ToothGrowth, aes(x=dose, fill=dose))+ 
  geom_bar() + 
  geom_bar(colour="black", show_guide=FALSE)

guides() : set or remove the legend for a specific aesthetic

It’s possible to use the function guides() to set or remove the legend of a particular aesthetic(fill, color, size, shape, etc).

mtcars data sets are used :

# Prepare the data : convert cyl and gear to factor variables
mtcars$cyl<-as.factor(mtcars$cyl)
mtcars$gear <- as.factor(mtcars$gear)
head(mtcars)

##                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
## Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
## Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
## Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
## Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
## Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
## Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

p <- ggplot(data = mtcars, 
    aes(x=mpg, y=wt, color=cyl, size=qsec, shape=gear))+
    geom_point()
# Print the plot without guide specification
p

# Change the legend position
p +theme(legend.position="bottom")

# Horizontal legend box
p +theme(legend.position="bottom", legend.box = "horizontal")

p+guides(color = guide_legend(order=1),
         size = guide_legend(order=2),
         shape = guide_legend(order=3))

qplot(data = mpg, x = displ, y = cty, size = hwy,
      colour = cyl, shape = drv) +
  guides(colour = guide_colourbar(order = 1),
         alpha = guide_legend(order = 2),
         size = guide_legend(order = 3))

p+guides(color = FALSE, size = FALSE)

# Remove legend for the point shape
p+scale_shape(guide=FALSE)
# Remove legend for size
p +scale_size(guide=FALSE)
# Remove legend for color
p + scale_color_manual(values=c('#999999','#E69F00','#56B4E9'),
                       guide=FALSE)

Infos

This analysis has been performed using R software (ver. 3.1.0) and ggplot2 (ver. 1.0.0)