R Graphics Cookbook, 2nd Edition

Chapter 4. Line Graphs

Line graphs are typically used for visualizing how one continuous variable, on the y-axis, changes in relation to another continuous variable, on the x-axis. Often the x variable represents time, but it may also represent some other continuous quantity; for example, the amount of a drug administered to experimental subjects.

As with bar graphs, there are exceptions. Line graphs can also be used with a discrete variable on the x-axis. This is appropriate when the variable is ordered (e.g., “small,” “medium,” “large”), but not when the variable is unordered (e.g., “cow,” “goose,” “pig”). Most of the examples in this chapter use a continuous x variable, but we’ll see one example where the variable is converted to a factor and thus treated as a discrete variable.

4.1 Making a Basic Line Graph

Problem

You want to make a basic line graph.

Solution

Use ggplot() with geom_line(), and specify which variables you mapped to x and y (Figure 4-1):

ggplot(BOD, aes(x = Time, y = demand)) +
  geom_line()

Figure 4-1. Basic line graph

Discussion

In this sample data set, the x variable, Time, is in one column and the y variable, demand, is in another:

BOD
#>   Time demand
#> 1    1    8.3
#> 2    2   10.3
#> 3    3   19.0
#> 4    4   16.0
#> 5    5   15.6
#> 6    7   19.8

Line graphs can be made with discrete (categorical) or continuous (numeric) variables on the x-axis. In the example here, the variable ...