12 Modify Legend

In this chapter, we customize legend guides: titles, labels and colorbars using the guide argument with guide_legend() and guide_colorbar().

Why this matters

Guides control title placement, label layout and colorbar sizing for publication-ready legends.

Common pitfall

Over-customizing direction, rows and justification in one guide hurts readability. Change one element at a time.

12.1 Customize legends with guides

In this section, we will learn to modify

title
label
and bar

So far, we have learnt to modify the components of a legend using scale_* family of functions. Now, we will use the guide argument and supply it values using the guide_legend() function.

Change the guide title

Title Alignment

The horizontal alignment of the title can be managed using the title.hjust argument. It can take any value between 0 and 1.

0 (left)
1 (right)

In the below example, we align the title to the center by assigning the value 0.5.

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(title = "Cylinders", title.hjust = 0.5))

Title Alignment (Vertical)

To manage the vertical alignment of the title, use title.vjust.

ggplot(mtcars) + geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    title = "Horsepower", title.position = "top", title.vjust = 1))

Title Position

The position of the title can be managed using title.position argument. It can be positioned at:

top
bottom
left
right

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(title = "Cylinders", title.hjust = 0.5, 
      title.position = "top"))

Change guide labels

Label Position

The position of the label can be managed using the label.position argument. It can be positioned at:

top
bottom
left
right

In the below example, we position the label at right.

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(label.position = "right"))

Label Alignment

The horizontal alignment of the label can be managed using the label.hjust argument. It can take any value between 0 and 1.

0 (left)
1 (right)

In the below example, we align the label to the center by assigning the value 0.5.

alignment
- 0 (left)
- 1 (right)

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(label.hjust = 0.5))

Labels Alignment (Vertical)

The vertical alignment of the label can be managed using the label.vjust argument.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    label.vjust = 0.8))

Direction

The direction of the label can be either horizontal or vertical and it can be set using the direction argument.

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(direction = "horizontal"))

Rows

The label can be spread across multiple rows using the nrow argument. In the below example, the label is spread across 2 rows.

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(nrow = 2))

Reverse

The order of the labels can be reversed using the reverse argument. We need to supply logical values i.e. either TRUE or FALSE. If TRUE, the order will be reversed.

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(reverse = TRUE))

Putting it all together

ggplot(mtcars) + geom_point(aes(disp, mpg, color = factor(cyl))) +
  scale_color_manual(values = c("red", "blue", "green"),
    guide = guide_legend(title = "Cylinders", title.hjust = 0.5, 
      title.position = "top", label.position = "right", 
      direction = "horizontal", label.hjust = 0.5, nrow = 2, reverse = TRUE)
  )

Change the legend color bar

So far we have looked at modifying components of the legend when it acts as a guide for color, fill or shape i.e. when the aesthetics have been mapped to a categorical variable. In this section, you will learn about guide_colorbar() which will allow us to modify the legend when the aesthetics are mapped to a continuous variable.

Plot

Let us start with a scatter plot examining the relationship between displacement and miles per gallon from the mtcars data set. We will map the color of the points to the hp variable.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp))

Width

The width of the bar can be modified using the barwidth argument. It is used inside the guide_colorbar() function which itself is supplied to the guide argument of scale_color_continuous().

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    barwidth = 10))

Height

Similarly, the height of the bar can be modified using the barheight argument.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    barheight = 3))

Bins

The nbin argument allows us to specify the number of bins in the bar.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    nbin = 4))

Ticks

The ticks of the bar can be removed using the ticks argument and setting it to FALSE.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    ticks = FALSE))

Upper/Lower Limits

The upper and lower limits of the bars can be drawn or undrawn using the draw.ulim and draw.llim arguments. They both accept logical values.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp)) +
  scale_color_continuous(guide = guide_colorbar(
    draw.ulim = TRUE, draw.llim = FALSE))

12.1.0.1 Guides: Color, Shape & Size

The guides() function can be used to create multiple legends to act as a guide for color, shape, size etc. as shown below. First, we map color, shape and size to different variables. Next, in the guides() function, we supply values to each of the above aesthetics to indicate the type of legend.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp, 
    size = qsec, shape = factor(gear))) + 
  guides(color = "colorbar", shape = "legend", size = "legend")

Guides: Title

To modify the components of the different legends, we must use the guide_* family of functions. In the below example, we use guide_colorbar() for the legend acting as guide for color mapped to a continuous variable and guide_legend() for the legends acting as guide for shape/size mapped to categorical variables.

ggplot(mtcars) +
  geom_point(aes(disp, mpg, color = hp, size = wt, shape = factor(gear))) + 
  guides(color = guide_colorbar(title = "Horsepower"),
    shape = guide_legend(title = "Gear"), size = guide_legend(title = "Weight")
  )

12.2 Exercises

Center a legend title with guide_legend(title.hjust = 0.5).
Move discrete legend labels with label.position.
Size a colorbar with barwidth/barheight for a continuous guide.

Worked solutions in solutions/legend.md (attempt first).

Where to go next

ggplot2 book scales and guides chapters