Bar graphs

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Bar Graphs

Definition

Bar graphs, also called bar charts, compare values between categories of data by using rectangular bars.

Bar charts might be a little easier to create than some other types of graphs that might take a long time to construct. The figure below shows a bar graph.

In this lesson, you will learn

  • How to read values on a bar graph
  • The 5 parts of a bar graph
  • The 4 types of bar graphs
  • How to answer questions using a bar graph

How to Read a Bar Graph

Vertical bar graph titled Days of Snow: December 2 days, January 6 days, February 14 days and March about 3 days

The information displayed in the bar graph above is straightforward. It tells how many days of snow there were during the months of December, January, February, and March.

For example, in December there were two days of snow and in February there were fourteen days of snow.

Remember

If the bar does not extend to the next line on the scale, just approximate the value.

For the month of March, the bar does not extend all the way to four. However, a close look shows that the bar is between 2 and 4. Therefore, a good approximation is 3, or 3 days of snow in March.

In the Days of Snow bar graph, how many days of snow were there in January?

Show answer

6 days. The top of the January bar is on the line for 6.

Parts of a Bar Graph

A bar graph has the following 5 parts:

  • Title
  • Categories
  • Scale
  • Labels
  • Rectangular bars

The title tells what the bar graph is about. For example, the title of the bar graph above is "Days of Snow".

The categories show the categorical data that you are trying to compare. For example, December, January, February, and March are the different categories you are trying to compare. Each category, such as January, is a value of the categorical variable. Categories are usually displayed on the x-axis.

The scale shows the amounts for the categories and the interval that is used to compare the categories. The scale is usually displayed on the y-axis.

There are two different labels on a bar graph, one for the categories and another one for the scale. The one for the categories tells you what the categories are about and the one for the scale shows what is being counted. For example, the label used for the categories is "Month" and the label used for the scale is "Number of Days of Snow".

Finally, we use rectangular bars of equal width to show frequencies. The height of the rectangular bar represents the frequency of each category.

In the Days of Snow bar graph, what is the interval of the scale?

Show answer

2. The numbers on the scale go up by 2: 0, 2, 4, 6, and so on.

Types of Bar Graphs

Here are the 4 different types of bar graphs:

  • Vertical bar graph
  • Horizontal bar graph
  • Grouped bar graph (also called multiple bar chart or clustered bar chart)
  • Stacked bar graph

The vertical bar graph is the most common type of bar graph. It is the one that you see in the figure above.

Horizontal Bar Graphs

The bars do not have to be vertical. Information can be displayed as well with horizontal bar graphs as shown below. In a horizontal bar graph, the different categories are displayed on the y-axis while the scale is displayed on the x-axis.

Horizontal bar graph titled Days of Snow with the months on the y-axis and the number of days on the x-axis

Grouped Bar Graphs

When each category can have more than one value, you could use a grouped bar graph to display the data. The grouped bar graph below shows the math scores for 4 students in the 8th grade. Notice that a grouped bar graph includes a key at the bottom of the graph and different colors to represent the different values in each category.

For example, the red bar is the score on the final exam. Eric got a 60 on the final exam.

Grouped bar graph of math scores for 4 students in 8th grade, with a pre-test, test and final exam bar for John, Sylvia, Eric and Mia

Stacked Bar Graphs

The stacked bar graph is similar to the grouped bar graph since each category can also have more than one value. However, with a stacked bar graph, just like the name implies, the values are stacked on top of each other.

In the graph below, the category called "Summer" has 3 values stacked on top of each other and the amount of money the family spent during the summer on Restaurant/Ice cream is 1200 dollars.

Stacked bar graph of some expenses of a family of 4 in spring, summer, fall and winter, split into travel, electricity/HVAC and restaurant/ice cream
Common mistake

Reading the top of a stacked piece as its value. In Summer, the red travel piece ends at about $6,800, but the family spent $5,000 on travel. A piece's value is its own height: where it ends minus where it starts.

How much did the family spend in all during the summer?

Show answer

$6,800. $1,200 + $600 + $5,000 = $6,800, which is the top of the Summer bar.

How to Read Information on Bar Graphs

Use the bar graph below to answer the following questions:

Bar graph titled Scores on a basic math test: 1 student scored 40, 2 scored 50, 4 scored 60, 3 scored 70, 6 scored 80 and 2 scored 90

a. What is the scale of the bar graph?

The scale is on the left of the graph and the scale goes from 1 to 7 with an interval of 1.

b. What is the title of the graph?

The title is "Scores on a basic math test".

c. How many students scored 80?

6 students scored 80.

d. How many students got 60 on the test?

4 students got 60.

e. How many students took the test?

1 student got 40, 2 students got 50, 4 students got 60, 3 students got 70, 6 students got 80, and 2 students got 90.

1 + 2 + 4 + 3 + 6 + 2 = 18

So, 18 students took the test.

f. Why is there no bar above 100?

There is no bar above 100 because no one got 100 on the test.

g. If the passing grade is 70, how many students passed?

3 students got 70, 6 students got 80, and 2 students got 90.

3 + 6 + 2 = 11. So 11 students passed the test.

Using the same bar graph, how many students scored less than 70?

Show answer

7 students. 1 student got 40, 2 got 50 and 4 got 60, and 1 + 2 + 4 = 7.

Summary

  • A bar graph compares values between categories by using rectangular bars.
  • The height (or length) of each bar shows the value for its category.
  • If a bar ends between two lines on the scale, approximate the value.
  • The 5 parts of a bar graph are the title, the categories, the scale, the labels and the bars.
  • The 4 types are vertical, horizontal, grouped and stacked bar graphs.
  • In a stacked bar graph, a piece's value is its own height, not where it ends.

Test Yourself: 10-Question Quiz

Now check what you learned. Every answer is explained, and each time you play you get new bar graphs.

Question 1 of 10Score: 0

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