4.1 Making a Basic Line Graph

4.1.1 Problem

You want to make a basic line graph.

4.1.2 Solution

Use ggplot() with geom_line(), and specify which variables you mapped to x and y (Figure 4.1):

Basic line graph

Figure 4.1: Basic line graph

4.1.3 Discussion

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

Line graphs can be made with discrete (categorical) or continuous (numeric) variables on the x-axis. In the example here, the variable demand is numeric, but it could be treated as a categorical variable by converting it to a factor with factor() (Figure 4.2). When the x variable is a factor, you must also use aes(group=1) to ensure that ggplot knows that the data points belong together and should be connected with a line (see Recipe 4.3 for an explanation of why group is needed with factors):

Basic line graph with a factor on the x-axis (notice that no space is allocated on the x-axis for 6)

Figure 4.2: Basic line graph with a factor on the x-axis (notice that no space is allocated on the x-axis for 6)

In the BOD data set there is no entry for Time = 6, so there is no level 6 when Time is converted to a factor. Factors hold categorical values, and in that context, 6 is just another value. It happens to not be in the data set, so there’s no space for it on the x-axis.

With ggplot2, the default y range of a line graph is just enough to include the y values in the data. For some kinds of data, it’s better to have the y range start from zero. You can use ylim() to set the range, or you can use expand_limits() to expand the range to include a value. This will set the range from zero to the maximum value of the demand column in BOD (Figure 4.3):

Line graph with manually set y range

Figure 4.3: Line graph with manually set y range

4.1.4 See Also

See Recipe 8.2 for more on controlling the range of the axes.