2.2 Creating a Line Graph

2.2.1 Problem

You want to create a line graph.

2.2.2 Solution

To make a line graph using plot() (Figure 2.3, left), pass it a vector of x values and a vector of y values, and use type = "l":

plot(pressure$temperature, pressure$pressure, type = "l")
Line graph with base graphics (left); With points and another line (right)Line graph with base graphics (left); With points and another line (right)

Figure 2.3: Line graph with base graphics (left); With points and another line (right)

To add points and/or multiple lines (Figure 2.3, right), first call plot() for the first line, then add points with points() and additional lines with lines():

plot(pressure$temperature, pressure$pressure, type = "l")
points(pressure$temperature, pressure$pressure)

lines(pressure$temperature, pressure$pressure/2, col = "red")
points(pressure$temperature, pressure$pressure/2, col = "red")

With ggplot2, you can get a similar result using geom_line() (Figure 2.4):

library(ggplot2)
ggplot(pressure, aes(x = temperature, y = pressure)) +
  geom_line()
Line graph with ggplot() (left); With points added (right)Line graph with ggplot() (left); With points added (right)

Figure 2.4: Line graph with ggplot() (left); With points added (right)

As with scatter plots, you can pass you data in vectors instead of in a data frame (but this will limit the things you can do later with the plot):

ggplot(pressure, aes(x = temperature, y = pressure)) +
  geom_line() +
  geom_point()

Note

It’s common with ggplot() to split the command on multiple lines, ending each line with a + so that R knows that the command will continue on the next line.

2.2.3 See Also

See Chapter 4 for more in-depth information about creating line graphs.