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.