Pie Charts in R
There are a number of important types of plots that are used in descriptive statistics. A common plot that is frequently used in newspapers and magazines is the pie plot, where the size of a "wedge of pie" help the reader visualize the percentage of data falling in a particular category. Take, for example, the responses of children who were asked their favorite color of M and M candies.
Note that the data in the table are percentages, summing to 100%.
A Simple Pie Chart
We'll first enter the data from the table above, storing the list in the variable mypie
You can obtain help on R's combine command by typing ?c at the prompt. This command is used to concatenate elements into a single list, particularly useful when entering data manually. You can see the contents of the variable by typing mypie and hitting the Enter key.
> mypie  40 30 20 10
It is a simple task to create a pie chart using the data stored in mypie.
The command pie(mypie) produces the pie chart shown in Figure 1.
Figure 1. A simple pie chart representing the percentages of children favoring red, blue, green and brown candies.
In the upcoming exercises, we'll explore further the capabilities of R's pie command. You can read more about this command by entering the command ?pie and reading the resulting help file.
Adding Labels to the Pie Chart
We can add "names" to the percentages of colored candies favored by the children.
The names command attaches a "name" to each piece of data. We can see the result of this command by typing the variable name (mypie) and hitting the Enter key.
> mypie Red Blue Green Brown 40 30 20 10
Note how each piece of data now has a "name" or "header." We can access individual elements of the list with R's indexing capability. For example, to access the first element of the list (R starts indexing at one), enter:
> mypie Red 40
However, since the data have "names" attached, we can access each data item through its "name."
> mypie['Red'] Red 40
The pie command knows how to apply these "names" to the pie chart.
When "names" are added to each piece of data (as we did to the variable mypie), R's pie command automatically adds these names as labels to the corresponding "slice of pie," as shown in Figure 2.
Figure 2. Adding labels to each "slice of pie."
Using Custom Colors
It won't do that the "Red" slice is colored "White." Let's see how we can use customized colors for each "slice of pie." First, we store the corresponding colors for each slice in the variable mycolors.
Next, we pass the colors in mycolors to the col argument of the function mypie.
This last command produces the pie chart shown in Figure 3.
Figure 3. Adding custom colors to each "slice of pie."
We hope you enjoyed this very short introducion to pie charts using R. To find out more about what can be accomplished with R's pie command, remember to type ?pie, hit Enter, and read the resulting help file.