Data Visualization using GGPlot2

Key R functions

• Key functions:
  • geom_path() connects the observations in the order in which they appear in the data.
  • geom_line() connects them in order of the variable on the x axis.
  • geom_step() creates a stairstep plot, highlighting exactly when changes occur.
• Key arguments to customize the plot: alpha, color, linetype and size

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, 4)

##   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, 4)

##   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

• 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 line plots

p <- ggplot(data = df, aes(x = dose, y = len, group = 1)) 
# Basic line plot with points
 p + geom_line() + geom_point()

# Change line type and color
p + geom_line(linetype = "dashed", color = "steelblue")+
  geom_point(color = "steelblue")

# Use geom_step()
p + geom_step() + geom_point()

Note that, the group aesthetic determines which cases are connected together.

Line plot with multiple groups

In the graphs below, line types and point shapes are controlled automatically by the levels of the variable supp:

p <- ggplot(df2, aes(x = dose, y = len, group = supp))
# Change line types and point shapes by groups
p + geom_line(aes(linetype = supp)) +
    geom_point(aes(shape = supp))

# Change line types, point shapes and colors
# Change color manually: custom color
p + geom_line(aes(linetype = supp, color = supp))+
    geom_point(aes(shape = supp, color = supp)) +
    scale_color_manual(values=c("#999999", "#E69F00"))

Line plot with a numeric x-axis

If the variable on x-axis is numeric, it can be useful to treat it as a continuous or a factor variable depending on what you want to do:

# Create some data
df3 <- data.frame(supp=rep(c("VC", "OJ"), each=3),
                dose=rep(c("0.5", "1", "2"),2),
                len=c(6.8, 15, 33, 4.2, 10, 29.5))
head(df3)

##   supp dose  len
## 1   VC  0.5  6.8
## 2   VC    1 15.0
## 3   VC    2 33.0
## 4   OJ  0.5  4.2
## 5   OJ    1 10.0
## 6   OJ    2 29.5

# x axis treated as continuous variable
df3$dose <- as.numeric(as.vector(df3$dose))
ggplot(data = df3, aes(x = dose, y = len, group = supp, color = supp)) +
  geom_line() + geom_point()

# Axis treated as discrete variable
df3$dose<-as.factor(df3$dose)
ggplot(data=df3, aes(x = dose, y = len, group = supp, color = supp)) +
  geom_line() + geom_point()

Line plot with dates on x-axis: Time series

economics time series data sets are used :

head(economics)

## # A tibble: 6 x 6
##   date         pce    pop psavert uempmed unemploy
##   <date>     <dbl>  <int>   <dbl>   <dbl>    <int>
## 1 1967-07-01  507. 198712    12.5     4.5     2944
## 2 1967-08-01  510. 198911    12.5     4.7     2945
## 3 1967-09-01  516. 199113    11.7     4.6     2958
## 4 1967-10-01  513. 199311    12.5     4.9     3143
## 5 1967-11-01  518. 199498    12.5     4.7     3066
## 6 1967-12-01  526. 199657    12.1     4.8     3018

Plots :

# Basic line plot
ggplot(data=economics, aes(x = date, y = pop))+
  geom_line()

# Plot a subset of the data
ss <- subset(economics, date > as.Date("2006-1-1"))
ggplot(data = ss, aes(x = date, y = pop)) + geom_line()

Change line size :

ggplot(data = economics, aes(x = date, y = pop)) +
  geom_line(aes(size = unemploy/pop))

Plot multiple time series data:

ggplot(economics, aes(x=date)) + 
  geom_line(aes(y = psavert), color = "darkred") + 
  geom_line(aes(y = uempmed), color="steelblue", linetype="twodash") 

# Area plot
ggplot(economics, aes(x=date)) + 
  geom_area(aes(y = psavert), fill = "#999999", 
            color = "#999999", alpha=0.5) + 
  geom_area(aes(y = uempmed), fill = "#E69F00",
            color = "#E69F00",  alpha=0.5) 

Conclusion

This article shows how to create line plots using the ggplot2 package.