Legends (ggplot2)

Table of contents

Problem

You want to modify the legend of a graph made with ggplot2.

Solution

Start with an example graph with the default options: 

library(ggplot2)
bp <- ggplot(data=PlantGrowth, aes(x=group, y=weight, fill=group)) + geom_boxplot()
bp

Removing the legend

bp + opts(legend.position="none")

Changing the order of items in the legend

This changes the order of items to trt1, ctrl, trt2:

bp + scale_fill_hue(breaks=c("trt1","ctrl","trt2"))

Depending on how the colors are specified, you may have to use a different scale, such as scale_fill_manual, scale_colour_hue, scale_colour_manual, scale_shape_discrete, scale_linetype_discrete, and so on.

Hiding the legend title

This will hide the legend title:

bp + opts(legend.title=theme_blank())

Modifying the text of legend titles and labels

There are two ways of changing the legend title and labels. The first way is to change data frame so that the factor has the desired form. The second way is to specify a scale to ggplot.

Changing the factor in the data frame

pg <- PlantGrowth    # Copy data into new data frame
# Rename the column and the values in the factor
names(pg)[names(pg)=="group"]  <- "Experimental Condition"
levels(pg)[levels(pg)=="ctrl"] <- "Control"
levels(pg)[levels(pg)=="trt1"] <- "Treatment 1"
levels(pg)[levels(pg)=="trt2"] <- "Treatment 2"

# The end product
pg
# weight Experimental Condition
#   4.17                   ctrl
#   5.58                   ctrl
#    ...
#   5.80                   trt2
#   5.26                   trt2

# Make the plot 
ggplot(data=pg, aes(x=`Experimental Condition`, y=weight, fill=`Experimental Condition`)) + geom_boxplot()

The legend title "Experimental Condtion" is long and it might look better if it were broken into two lines, but this doesn't work very well with this method, since you would have to put a newline character in the name of the column. The other method, with scales, is generally a better way to do this.

Also note the use of backticks instead of quotes. They are necessary because of the spaces in the variable name.

Using scales

The other method is to use a scale with ggplot.

With color fills

Because group, the variable in the legend, is mapped to the color fill, it is necessary to use scale_fill_xxx, where xxx is a method of mapping each factor level of group to different colors. The default is to use a different hue on the color wheel for each factor level, but it is also possible to manually specify the colors for each level.

bp + scale_fill_hue(name="Experimental\nCondition")

bp + scale_fill_hue(name="Experimental\nCondition",
                    breaks=c("ctrl", "trt1", "trt2"),
                    labels=c("Control", "Treatment 1", "Treatment 2"))

# Using a manual scale instead of hue
bp + scale_fill_manual(values=c("#999999", "#E69F00", "#56B4E9"), 
                       name="Experimental\nCondition",
                       breaks=c("ctrl", "trt1", "trt2"),
                       labels=c("Control", "Treatment 1", "Treatment 2"))

(Note that the x-axis labels did not change. See ../Axes (ggplot2) for more information.)

With lines and points

If you use a line graph, you will probably need to use scale_colour_xxx and/or scale_shape_xxx instead of scale_fill_xxx. colour maps to the colors of lines and points, while fill maps to the color of area fills. shape maps to the shapes of points.

We will use a different data set for the line graphs here because the PlantGrowth data set does not work well with a line graph.

# A different data set
df1 <- data.frame(sex        = factor(c("Female","Female","Male","Male")),
                  time       = factor(c("Lunch","Dinner","Lunch","Dinner"), levels=c("Lunch","Dinner")),
                  total_bill = c(13.53, 16.81, 16.24, 17.42))

# A basic graph
lp <- ggplot(data=df1, aes(x=time, y=total_bill, group=sex, shape=sex)) + geom_line() + geom_point()
lp

# Change the legend
lp + scale_shape_discrete(name  ="Payer",
                          breaks=c("Female", "Male"),
                          labels=c("Woman", "Man"))

If you use both colour and shape, they both need to be given scale specifications. Otherwise there will be two two separate legends.

# Specify colour and shape
lp1 <- ggplot(data=df1, aes(x=time, y=total_bill, group=sex, shape=sex, colour=sex)) + geom_line() + geom_point()
lp1

# Here's what happens if you just specify colour
lp1 + scale_colour_hue(name  ="Payer",
                       breaks=c("Female", "Male"),
                       labels=c("Woman", "Man"))

# Specify both colour and shape
lp1 + scale_colour_hue(name  ="Payer",
                       breaks=c("Female", "Male"),
                       labels=c("Woman", "Man")) +
      scale_shape_discrete(name  ="Payer",
                       breaks=c("Female", "Male"),
                       labels=c("Woman", "Man"))

Kinds of scales

There are many kinds of scales. They take the form scale_xxx_yyy. Here are some commonly-used values of xxx and yyy:

xxx Description
colour Color of lines and points
fill Color of area fills (e.g. bar graph)
linetype Solid/dashed/dotted lines
shape Shape of points
size Size of points
alpha Opacity/transparency
yyy Description
hue Equally-spaced colors from the color wheel
manual Manually-specified values (e.g., colors, point shapes, line types)
gradient Color gradient
grey Shades of grey
discrete Discrete values (e.g., colors, point shapes, line types, point sizes)
continuous Continuous values (e.g., alpha, colors, point sizes)

Modifying the appearance of the legend title and labels

bp + opts(legend.title = theme_text(colour="blue", size=16, face="bold"))

# Adjust the horizontal position of the title
bp + opts(legend.title = theme_text(colour="blue", size=16, hjust=-.1, face="bold"))

bp + opts(legend.text = theme_text(colour="blue", size = 16, face = "bold"))

Modifying the legend box

By default, the legend will not have a box around it. To add a box and modify its properties:

bp + opts(legend.background = theme_rect())
bp + opts(legend.background = theme_rect(fill="gray90", size=.5, linetype="dotted"))

Changing the position of the legend

Position legend outside the plotting area (left/right/top/bottom):

bp + opts(legend.position="left")

It is also possible to position the legend inside the plotting area. Note that the numeric position below is relative to the entire area, including titles and labels, not just the plotting area.

# Position legend in graph, from 0,0 (bottom left) to 1,1 (top right)
bp + opts(legend.position=c(.8, .35))

# Set the "anchoring point" of the legend (left/right/centre)
bp + opts(legend.justification="left", legend.position=c(0.5, 0.5))
bp + opts(legend.justification="right", legend.position=c(0.5, 0.5))

Hiding slashes in the legend

If you make bar graphs with an outline (by setting colour="black"), it will draw a slash through the colors in the legend. There is not a built-in way to remove the slashes, but it is possible to cover them up.

# No outline
ggplot(data=PlantGrowth, aes(x=group, fill=group)) + geom_bar()

# Add outline, but slashes appear in legend
ggplot(data=PlantGrowth, aes(x=group, fill=group)) + geom_bar(colour="black")

# A hack to hide the slashes: first graph the bars with no outline and add the legend,
# then graph the bars again with outline, but with a blank legend.
ggplot(data=PlantGrowth, aes(x=group, fill=group)) + geom_bar() + geom_bar(colour="black", legend=FALSE)

Notes

For further information, see: [https://github.com/hadley/ggplot2/wiki/Legend-Attributes]