Explainer: Double Bar Graphs

In this explainer, we will learn how to use a double bar graph to display two sets of related data using bars of different colors and heights.

A helpful way to display data for a nonnumerical (categorical) variable is in a bar graph. For example, suppose we have collected data on the preferred platform used by a group of core gamers. The variable is “platform,” which has 3 categories: “smart phone,” “console,” and “PC.” And we have counted the number of gamers who chose each category.

The height of a bar, which we can read off the vertical axis, is the number of gamers who chose that option. So, for example, we can see that as the height of the bar above the “PC” option is 80, we know that 80 gamers preferred a PC for gaming.

When we have two categorical variables, we can use a double (or multiple) bar graph to gain information about the two variables. Let us look at an example.

Example 1: Constructing and Reading Information from a Double Bar Graph

The data in the table below represents the preferred gaming platform for a group of gamers broken down by gender.

  1. Illustrate the data in a double bar graph.
  2. From your bar graph, determine which is the most popular platform among female gamers.

Answer

Part 1

To illustrate the data in a double bar graph, as with the single bar graph, we put the “platform” categories on the horizontal axis and the frequency (number of gamers) on the vertical axis. We note that the highest frequency in the table is 50, corresponding to male gamers who prefer smartphones as their gaming platform. We use a slightly larger number than this, say, 60, as our maximum vertical marker. Above each “platform” category, we place two bars: one for male gamers and one for female gamers.

The height of each bar represents the number of gamers who chose that category of platform. The bars for female and male gamers are different colors so that we can easily distinguish between them, and we have a key or legend on the graph telling us which color corresponds to which gender.

Part 2

To determine from the bar graph which is the most popular platform among female gamers, we simply look for the tallest green bar, since the green bars represent the female gamers.

The highest green bar is above the “smart phone” category. Hence, the most popular platform for gaming among female gamers was “smart phone.”

In our next example, we will use a double bar graph to gain information from data.

Example 2: Reading Information from a Double Bar Graph

The bar graph shows the number of runs that two teams achieved in each game of a baseball playoff. How many runs did the Hawks score in game 3?

Answer

To find how many runs the Hawks scored in game 3, we note first from the key that the Hawks’data is represented by the blue bars. The blue bar above game 3 on the horizontal axis has a height of 7.