Chapter 10 Customizing Graphs

Graph defaults are fine for quick data exploration, but when you want to publish your results to a blog, paper, article or poster, you’ll probably want to customize the results. Customization can improve the clarity and attractiveness of a graph.

This chapter describes how to customize a graph’s axes, gridlines, colors, fonts, labels, and legend. It also describes how to add annotations (text and lines).

10.1 Axes

The x-axis and y-axis represent numeric, categorical, or date values. You can modify the default scales and labels with the functions below.

10.1.1 Quantitative axes

A quantitative axis is modified using the scale_x_continuous or scale_y_continuous function.

Options include

  • breaks - a numeric vector of positions
  • limits - a numeric vector with the min and max for the scale
# customize numerical x and y axes
library(ggplot2)
ggplot(mpg, aes(x=displ, y=hwy)) +
  geom_point() +
  scale_x_continuous(breaks = seq(1, 7, 1), 
                     limits=c(1, 7)) +
  scale_y_continuous(breaks = seq(10, 45, 5), 
                     limits=c(10, 45))
Customized quantitative axes

Figure 10.1: Customized quantitative axes

10.1.1.1 Numeric formats

The scales package provides a number of functions for formatting numeric labels. Some of the most useful are

  • dollar
  • comma
  • percent

Let’s demonstrate these functions with some synthetic data.

# create some data
set.seed(1234)
df <- data.frame(xaxis = rnorm(50, 100000, 50000),
                 yaxis = runif(50, 0, 1),
                 pointsize = rnorm(50, 1000, 1000))
library(ggplot2)

# plot the axes and legend with formats
ggplot(df, aes(x = xaxis, 
               y = yaxis, 
               size=pointsize)) + 
  geom_point(color = "cornflowerblue",
             alpha = .6) +
  scale_x_continuous(label = scales::comma) +
  scale_y_continuous(label = scales::percent) +
  scale_size(range = c(1,10), # point size range
             label = scales::dollar)
Formatted axes

Figure 10.2: Formatted axes

To format currency values as euros, you can use

label = scales::dollar_format(prefix = "", suffix = "\u20ac").

10.1.2 Categorical axes

A categorical axis is modified using the scale_x_discrete or scale_y_discrete function.

Options include

  • limits - a character vector (the levels of the quantitative variable in the desired order)
  • labels - a character vector of labels (optional labels for these levels)
library(ggplot2)
# customize categorical x axis
ggplot(mpg, aes(x = class)) +
  geom_bar(fill = "steelblue") +
  scale_x_discrete(limits = c("pickup", "suv", "minivan", 
                             "midsize", "compact", "subcompact", 
                             "2seater"),
                   labels = c("Pickup\nTruck", "Sport Utility\nVehicle", 
                              "Minivan", "Mid-size", "Compact", 
                              "Subcompact", "2-Seater"))
Customized categorical axis

Figure 10.3: Customized categorical axis

10.1.3 Date axes

A date axis is modified using the scale_x_date or scale_y_date function.

Options include

  • date_breaks - a string giving the distance between breaks like “2 weeks” or “10 years”
  • date_labels - A string giving the formatting specification for the labels

The table below gives the formatting specifications for date values.

Symbol Meaning Example
%d day as a number (0-31) 01-31
%a abbreviated weekday Mon
%A unabbreviated weekday Monday
%m month (00-12) 00-12
%b abbreviated month Jan
%B unabbreviated month January
%y 2-digit year 07
%Y 4-digit year 2007
library(ggplot2)
# customize date scale on x axis
ggplot(economics, aes(x = date, y = unemploy)) +
  geom_line(color="darkgreen") +
  scale_x_date(date_breaks = "5 years",
               date_labels = "%b-%y")
Customized date axis

Figure 10.4: Customized date axis

Here is a help sheet for modifying scales developed from the online help.

10.2 Colors

The default colors in ggplot2 graphs are functional, but often not as visually appealing as they can be. Happily this is easy to change.

Specific colors can be

  • specified for points, lines, bars, areas, and text, or
  • mapped to the levels of a variable in the dataset.

10.2.1 Specifying colors manually

To specify a color for points, lines, or text, use the color = "colorname" option in the appropriate geom. To specify a color for bars and areas, use the fill = "colorname" option.

Examples:

  • geom_point(color = "blue")
  • geom_bar(fill = "steelblue")

Colors can be specified by name or hex code.

To assign colors to the levels of a variable, use the scale_color_manual and scale_fill_manual functions. The former is used to specify the colors for points and lines, while the later is used for bars and areas.

Here is an example, using the diamonds dataset that ships with ggplot2. The dataset contains the prices and attributes of 54,000 round cut diamonds.

# specify fill color manually
library(ggplot2)
ggplot(diamonds, aes(x = cut, fill = clarity)) +
  geom_bar() +
  scale_fill_manual(values = c("darkred", "steelblue", 
                               "darkgreen", "gold",
                               "brown", "purple", 
                               "grey", "khaki4"))
Manual color selection

Figure 10.5: Manual color selection

If you are aesthetically challenged like me, an alternative is to use a predefined palette.

10.2.2 Color palettes

There are many predefined color palettes available in R.

10.2.2.1 RColorBrewer

The most popular alternative palettes are probably the ColorBrewer palettes.

RColorBrewer palettes

Figure 10.6: RColorBrewer palettes

You can specify these palettes with the scale_color_brewer and scale_fill_brewer functions.

# use an ColorBrewer fill palette
ggplot(diamonds, aes(x = cut, fill = clarity)) +
  geom_bar() +
  scale_fill_brewer(palette = "Dark2") 
Using RColorBrewer

Figure 10.7: Using RColorBrewer

Adding direction = -1 to these functions reverses the order of the colors in a palette.

10.2.2.2 Viridis

The viridis palette is another popular choice.

For continuous scales use

  • scale_fill_viridis_c
  • scale_color_viridis_c

For discrete (categorical scales) use

  • scale_fill_viridis_d
  • scale_color_viridis_d
# Use a viridis fill palette
ggplot(diamonds, aes(x = cut, fill = clarity)) +
  geom_bar() + 
  scale_fill_viridis_d()
Using the viridis palette

Figure 10.8: Using the viridis palette

10.2.2.3 Other palettes

Other palettes to explore include dutchmasters, ggpomological, LaCroixColoR, nord, ochRe, palettetown, pals, rcartocolor, and wesanderson.

If you want to explore all the palette options (or nearly all), take a look at the paletter package.

To learn more about color specifications, see the R Cookpage page on ggplot2 colors. Also see the color choice advice in this book.

10.3 Points & Lines

10.3.1 Points

For ggplot2 graphs, the default point is a filled circle. To specify a different shape, use the shape = # option in the geom_point function. To map shapes to the levels of a categorical variable use the shape = variablename option in the aes function.

Examples:

  • geom_point(shape = 1)
  • geom_point(aes(shape = sex))

Availabe shapes are given in the table below.

Point shapes

Figure 10.9: Point shapes

Shapes 21 through 26 provide for both a fill color and a border color.

10.3.2 Lines

The default line type is a solid line. To change the linetype, use the linetype = # option in the geom_line function. To map linetypes to the levels of a categorical variable use the linetype = variablename option in the aes function.

Examples:

  • geom_line(linetype = 1)
  • geom_line(aes(linetype = sex))

Availabe linetypes are given in the table below.

Linetypes

Figure 10.10: Linetypes

10.4 Fonts

R does not have great support for fonts, but with a bit of work, you can change the fonts that appear in your graphs. First you need to install and set-up the extrafont package.

# one time install
install.packages("extrafont")
library(extrafont)
font_import()

# see what fonts are now available
fonts()

Apply the new font(s) using the text option in the theme function.

# specify new font
library(extrafont)
ggplot(mpg, aes(x = displ, y=hwy)) +
  geom_point() +
  labs(title = "Diplacement by Highway Mileage",
       subtitle = "MPG dataset") +
  theme(text = element_text(size = 16, family = "Comic Sans MS"))
Alternative fonts

Figure 10.11: Alternative fonts

To learn more about customizing fonts, see Working with R, Cairo graphics, custom fonts, and ggplot.

10.5 Legends

In ggplot2, legends are automatically created when variables are mapped to color, fill, linetype, shape, size, or alpha.

You have a great deal of control over the look and feel of these legends. Modifications are usually made through the theme function and/or the labs function. Here are some of the most sought after.

10.5.1 Legend location

The legend can appear anywhere in the graph. By default, it’s placed on the right. You can change the default with

theme(legend.position = position)

where

Position Location
“top” above the plot area
“right” right of the plot area
“bottom” below the plot area
“left” left of the plot area
c(x, y) within the plot area. The x and y values must range between 0 and 1. c(0,0) represents (left, bottom) and c(1,1) represents (right, top).
“none” suppress the legend

For example, to place the legend at the top, use the following code.

# place legend on top
ggplot(mpg, 
       aes(x = displ, y=hwy, color = class)) +
  geom_point(size = 4) +
  labs(title = "Diplacement by Highway Mileage") + 
  theme_minimal() +
  theme(legend.position = "top") 
Moving the legend to the top

Figure 10.12: Moving the legend to the top

10.5.2 Legend title

You can change the legend title through the labs function. Use color, fill, size, shape, linetype, and alpha to give new titles to the corresponding legends.

The alignment of the legend title is controlled through the legend.title.align option in the theme function. (0=left, 0.5=center, 1=right)

# change the default legend title
ggplot(mpg, 
       aes(x = displ, y=hwy, color = class)) +
  geom_point(size = 4) +
  labs(title = "Diplacement by Highway Mileage",
       color = "Automobile\nClass") + 
  theme_minimal() +
  theme(legend.title.align=0.5)
Changing the legend title

Figure 10.13: Changing the legend title

See Hadley Wickam’s legend attributes for more details.

10.6 Labels

Labels are a key ingredient in rendering a graph understandable. They’re are added with the labs function. Available options are given below.

option Use
title main title
subtitle subtitle
caption caption (bottom right by default)
x horizontal axis
y vertical axis
color color legend title
fill fill legend title
size size legend title
linetype linetype legend title
shape shape legend title
alpha transparency legend title
size size legend title

For example

# add plot labels
ggplot(mpg, 
       aes(x = displ, y=hwy, 
           color = class,
           shape = factor(year))) +
  geom_point(size = 3, 
             alpha = .5) +
  labs(title = "Mileage by engine displacement",
       subtitle = "Data from 1999 and 2008",
       caption = "Source: EPA (http://fueleconomy.gov)",
       x = "Engine displacement (litres)",
       y = "Highway miles per gallon",
       color = "Car Class",
       shape = "Year") + 
  theme_minimal()
Graph with labels

Figure 10.14: Graph with labels

This is not a great graph - it is too busy, making the identification of patterns difficult. It would better to facet the year variable, the class variable or both. Trend lines would also be helpful.

10.7 Annotations

Annotations are addition information added to a graph to highlight important points.

10.7.1 Adding text

There are two primary reasons to add text to a graph.

One is to identify the numeric qualities of a geom. For example, we may want to identify points with labels in a scatterplot, or label the heights of bars in a bar chart.

Another reason is to provide additional information. We may want to add notes about the data, point out outliers, etc.

10.7.1.1 Labeling values

Consider the following scatterplot, based on the car data in the mtcars dataset.

# basic scatterplot
data(mtcars)
ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point()
Simple scatterplot

Figure 10.15: Simple scatterplot

Let’s label each point with the name of the car it represents.

# scatterplot with labels
data(mtcars)
ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  geom_text(label = row.names(mtcars))
Scatterplot with labels

Figure 10.16: Scatterplot with labels

The overlapping labels make this chart difficult to read. There is a package called ggrepel that can help us here.

# scatterplot with non-overlapping labels
data(mtcars)
library(ggrepel)
ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  geom_text_repel(label = row.names(mtcars), 
                  size=3)
Scatterplot with non-overlapping labels

Figure 10.17: Scatterplot with non-overlapping labels

Much better.

Adding labels to bar charts is covered in the aptly named labeling bars section.

10.7.1.2 Adding additional information

We can place text anywhere on a graph using the annotate function. The format is

annotate("text", 
         x, y, 
         label = "Some text", 
         color = "colorname", 
         size=textsize)

where x and y are the coordinates on which to place the text. The color and size parameters are optional.

By default, the text will be centered. Use hjust and vjust to change the alignment.

  • hjust 0 = left justified, 0.5 = centered, and 1 = right centered.
  • vjust 0 = above, 0.5 = centered, and 1 = below.

Continuing the previous example.

# scatterplot with explanatory text
data(mtcars)
library(ggrepel)
txt <- paste("The relationship between car weight",
              "and mileage appears to be roughly linear",
              sep = "\n")
ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point(color = "red") +
  geom_text_repel(label = row.names(mtcars), 
                  size=3) +
  ggplot2::annotate("text", 
                    6, 30, 
                    label=txt,
                    color = "red",
                    hjust = 1) +
  theme_bw()
Scatterplot with arranged labels

Figure 10.18: Scatterplot with arranged labels

See this blog post for more details.

10.7.2 Adding lines

Horizontal and vertical lines can be added using:

  • geom_hline(yintercept = a)
  • geom_vline(xintercept = b)

where a is a number on the y-axis and b is a number on the x-axis respectively. Other option include linetype and color.

# add annotation line and text label
min_cty <- min(mpg$cty)
mean_hwy <- mean(mpg$hwy)
ggplot(mpg, 
       aes(x = cty, y=hwy, color=drv)) +
  geom_point(size = 3) +
  geom_hline(yintercept = mean_hwy,
             color = "darkred",
             linetype = "dashed") +
  ggplot2::annotate("text", 
           min_cty, 
           mean_hwy + 1, 
           label = "Mean",
           color = "darkred") +
  labs(title = "Mileage by drive type",
       x = "City miles per gallon",
       y = "Highway miles per gallon",
       color = "Drive")
Graph with line annotation

Figure 10.19: Graph with line annotation

We could add a vertical line for the mean city miles per gallon as well. In any case, always label annotation lines in some way. Otherwise the reader will not know what they mean.

10.7.3 Highlighting a single group

Sometimes you want to highlight a single group in your graph. The gghighlight function in the gghighlight package is designed for this.

Here is an example with a scatterplot.

# highlight a set of points
library(ggplot2)
library(gghighlight)
ggplot(mpg, aes(x = cty, y = hwy)) +
  geom_point(color = "red",
             size=2) +
  gghighlight(class == "midsize")
Highlighting a group

Figure 10.20: Highlighting a group

Below is an example with a bar chart.

# highlight a single bar
library(gghighlight)
ggplot(mpg, aes(x = class)) +
  geom_bar(fill = "red") +
  gghighlight(class == "midsize")
Highlighting a group

Figure 10.21: Highlighting a group

There is nothing here that could not be done with base graphics, but it is more convenient.

10.8 Themes

ggplot2 themes control the appearance of all non-data related components of a plot. You can change the look and feel of a graph by altering the elements of its theme.

10.8.1 Altering theme elements

The theme function is used to modify individual components of a theme.

The parameters of the theme function are described in a cheatsheet developed from the online help.

Consider the following graph. It shows the number of male and female faculty by rank and discipline at a particular university in 2008-2009. The data come from the Salaries for Professors dataset.

# create graph
data(Salaries, package = "carData")
p <- ggplot(Salaries, aes(x = rank, fill = sex)) +
  geom_bar() +
  facet_wrap(~discipline) +
  labs(title = "Academic Rank by Gender and Discipline",
       x = "Rank",
       y = "Frequency",
       fill = "Gender")
p
Graph with default theme

Figure 10.22: Graph with default theme

Let’s make some changes to the theme.

  • Change label text from black to navy blue
  • Change the panel background color from grey to white
  • Add solid grey lines for major y-axis grid lines
  • Add dashed grey lines for minor y-axis grid lines
  • Eliminate x-axis grid lines
  • Change the strip background color to white with a grey border

Using the cheat sheet gives us

p +
  theme(text = element_text(color = "navy"),
        panel.background = element_rect(fill = "white"),
        panel.grid.major.y = element_line(color = "grey"),
        panel.grid.minor.y = element_line(color = "grey", 
                                          linetype = "dashed"),
        panel.grid.major.x = element_blank(),
        panel.grid.minor.x = element_blank(),
        strip.background = element_rect(fill = "white", color="grey"))
Graph with modified theme

Figure 10.23: Graph with modified theme

Wow, this looks pretty awful, but you get the idea.

10.8.1.1 ggThemeAssist

If you would like to create your own theme using a GUI, take a look at ggThemeAssist. After you install the package, a new menu item will appear under Addins in RStudio.

ggThemeAssist Highlight the code that creates your graph, then choose the ggThemeAssist option from the Addins drop-down menu. You can change many of the features of your theme using point-and-click. When you’re done, the theme code will be appended to your graph code.

10.8.2 Pre-packaged themes

I’m not a very good artist (just look at the last example), so I often look for pre-packaged themes that can be applied to my graphs. There are many available.

Some come with ggplot2. These include theme_classic, theme_dark, theme_gray, theme_grey, theme_light theme_linedraw, theme_minimal, and theme_void. We’ve used theme_minimal often in this book. Others are available through add-on packages.

10.8.2.1 ggthemes

The ggthemes package come with 19 themes.

Theme Description
theme_base Theme Base
theme_calc Theme Calc
theme_economist ggplot color theme based on the Economist
theme_economist_white ggplot color theme based on the Economist
theme_excel ggplot color theme based on old Excel plots
theme_few Theme based on Few’s “Practical Rules for Using Color in Charts”
theme_fivethirtyeight Theme inspired by fivethirtyeight.com plots
theme_foundation Foundation Theme
theme_gdocs Theme with Google Docs Chart defaults
theme_hc Highcharts JS theme
theme_igray Inverse gray theme
theme_map Clean theme for maps
theme_pander A ggplot theme originated from the pander package
theme_par Theme which takes its values from the current ‘base’ graphics parameter values in ‘par’.
theme_solarized ggplot color themes based on the Solarized palette
theme_solarized_2 ggplot color themes based on the Solarized palette
theme_solid Theme with nothing other than a background color
theme_stata Themes based on Stata graph schemes
theme_tufte Tufte Maximal Data, Minimal Ink Theme
theme_wsj Wall Street Journal theme

To demonstrate their use, we’ll first create and save a graph.

# create basic plot
library(ggplot2)
p <- ggplot(mpg, 
            aes(x = displ, y=hwy, 
                color = class)) +
  geom_point(size = 3, 
             alpha = .5) +
  labs(title = "Mileage by engine displacement",
       subtitle = "Data from 1999 and 2008",
       caption = "Source: EPA (http://fueleconomy.gov)",
       x = "Engine displacement (litres)",
       y = "Highway miles per gallon",
       color = "Car Class") 

# display graph
p
Default theme

Figure 10.24: Default theme

Now let’s apply some themes.

# add economist theme
library(ggthemes)
p + theme_economist() 
Economist theme

Figure 10.25: Economist theme

# add fivethirtyeight theme
p + theme_fivethirtyeight()
Five Thirty Eight theme

Figure 10.26: Five Thirty Eight theme

# add wsj theme
p + theme_wsj(base_size=8)
Wall Street Journal theme

Figure 10.27: Wall Street Journal theme

By default, the font size for the wsj theme is usually too large. Changing the base_size option can help.

Each theme also comes with scales for colors and fills. In the next example, both the few theme and colors are used.

# add few theme
p + theme_few() + scale_color_few()
Few theme and colors

Figure 10.28: Few theme and colors

Try out different themes and scales to find one that you like.

10.8.2.2 hrbrthemes

The hrbrthemes package is focused on typography-centric themes. The results are charts that tend to have a clean look.

Continuing the example plot from above

# add few theme
library(hrbrthemes)
p + theme_ipsum()
Ipsum theme

Figure 10.29: Ipsum theme

See the hrbrthemes homepage for additional examples.

10.8.2.3 ggthemer

The ggthemer package offers a wide range of themes (17 as of this printing).

The package is not available on CRAN and must be installed from GitHub.

# one time install
install.packages("devtools")
devtools::install_github('cttobin/ggthemr')

The functions work a bit differently. Use the ggthemr("themename") function to set future graphs to a given theme. Use ggthemr_reset() to return future graphs to the ggplot2 default theme.

Current themes include flat, flat dark, camoflauge, chalk, copper, dust, earth, fresh, grape, grass, greyscale, light, lilac, pale, sea, sky, and solarized.

# set graphs to the flat dark theme
library(ggthemr)
ggthemr("flat dark")
p
Ipsum theme

Figure 10.30: Ipsum theme

ggthemr_reset()

I would not actually use this theme for this particular graph. It is difficult to distinguish colors. Which green represents compact cars and which represents subcompact cars?

Select a theme that best conveys the graph’s information to your audience.