Data Visualization using GGPlot2

Key R functions

• Key function: geom_col() for creating bar plots. The heights of the bars represent values in the data.
• Key arguments to customize the plot:
  • color, fill: bar border and fill color
  • width: bar width

Data preparation

We’ll create two data frames derived from the ToothGrowth datasets.

df <- data.frame(dose=c("D0.5", "D1", "D2"),
                len=c(4.2, 10, 29.5))

head(df)

##   dose  len
## 1 D0.5  4.2
## 2   D1 10.0
## 3   D2 29.5

df2 <- data.frame(supp=rep(c("VC", "OJ"), each=3),
                dose=rep(c("D0.5", "D1", "D2"),2),
                len=c(6.8, 15, 33, 4.2, 10, 29.5))

head(df2)

##   supp dose  len
## 1   VC D0.5  6.8
## 2   VC   D1 15.0
## 3   VC   D2 33.0
## 4   OJ D0.5  4.2
## 5   OJ   D1 10.0
## 6   OJ   D2 29.5

• len: Tooth length
• dose: Dose in milligrams (0.5, 1, 2)
• supp: Supplement type (VC or OJ)

Loading required R package

Load the ggplot2 package and set the default theme to theme_classic() with the legend at the top of the plot:

library(ggplot2)
theme_set(
  theme_classic() +
    theme(legend.position = "top")
  )

Basic barplots

We start by creating a simple barplot (named f) using the df data set:

f <- ggplot(df, aes(x = dose, y = len))

# Basic bar plot
f + geom_col()

# Change fill color and add labels at the top (vjust = -0.3)
f + geom_col(fill = "#0073C2FF") +
  geom_text(aes(label = len), vjust = -0.3)
   
# Label inside bars, vjust = 1.6
f + geom_col(fill = "#0073C2FF")+
  geom_text(aes(label = len), vjust = 1.6, color = "white")

Note that, it’s possible to change the width of bars using the argument width (e.g.: width = 0.5)

Change barplot colors by groups

We’ll change the barplot line and fill color by the variable dose group levels. To set a palette of custom color the function scale_color_manual() is used.

# Change barplot line colors by groups
f + geom_col(aes(color = dose), fill = "white") +
  scale_color_manual(values = c("#00AFBB", "#E7B800", "#FC4E07"))

# Change barplot fill colors by groups
f + geom_col(aes(fill = dose)) +
  scale_fill_manual(values = c("#00AFBB", "#E7B800", "#FC4E07"))

Barplot with multiple groups

Create stacked and dodged bar plots. Use the functions scale_color_manual() and scale_fill_manual() to set manually the bars border line colors and area fill colors.

# Stacked bar plots of y = counts by x = cut,
# colored by the variable color
ggplot(df2, aes(x = dose, y = len)) +
  geom_col(aes(color = supp, fill = supp), position = position_stack()) +
  scale_color_manual(values = c("#0073C2FF", "#EFC000FF"))+
  scale_fill_manual(values = c("#0073C2FF", "#EFC000FF"))

# Use position = position_dodge() 
p <- ggplot(df2, aes(x = dose, y = len)) +
  geom_col(aes(color = supp, fill = supp), position = position_dodge(0.8), width = 0.7) +
  scale_color_manual(values = c("#0073C2FF", "#EFC000FF"))+
  scale_fill_manual(values = c("#0073C2FF", "#EFC000FF"))
p

Add labels to a dodged barplot:

p + geom_text(
  aes(label = len, group = supp), 
  position = position_dodge(0.8),
  vjust = -0.3, size = 3.5
)

Add labels to a stacked bar plots. 4 steps required to compute the position of text labels:

• Group the data by the dose variable
• Sort the data by dose and supp columns. As position_stack() reverse the group order, supp column should be sorted in descending order.
• Calculate the cumulative sum of len for each dose category. Used as the y coordinates of labels. To put the label in the middle of the bars, we’ll use cumsum(len) - 0.5 * len.
• Create the bar graph and add labels

# Arrange/sort and compute cumulative summs
library(dplyr)
 df2 <- df2 %>%
  group_by(dose) %>%
  arrange(dose, desc(supp)) %>%
  mutate(lab_ypos = cumsum(len) - 0.5 * len) 
df2

## # A tibble: 6 x 4
## # Groups:   dose [3]
##   supp  dose    len lab_ypos
##   <fct> <fct> <dbl>    <dbl>
## 1 VC    D0.5    6.8      3.4
## 2 OJ    D0.5    4.2      8.9
## 3 VC    D1     15        7.5
## 4 OJ    D1     10       20  
## 5 VC    D2     33       16.5
## 6 OJ    D2     29.5     47.8

# Create stacked bar graphs with labels
ggplot(data = df2, aes(x = dose, y = len)) +
  geom_col(aes(fill = supp), width = 0.7)+
  geom_text(aes(y = lab_ypos, label = len, group =supp), color = "white") +
  scale_color_manual(values = c("#0073C2FF", "#EFC000FF"))+
  scale_fill_manual(values = c("#0073C2FF", "#EFC000FF"))

Conclusion

This article describes how to create and customize barplot using the ggplot2 R package.