Reading and Drawing Bar Graphs

A bar graph (also called a bar chart or bar diagram) uses rectangular bars to compare quantities across different categories. One axis lists the categories (like months, colors, or countries), and the other axis shows a numerical scale. The length or height of each bar tells you the value for that category. Bar graphs are one of the most common ways to display categorical data, and knowing how to read a bar graph and how to draw a bar graph step by step is a core statistics skill.

Before reading or drawing any bar graph, you should be able to identify its parts:

• Title: tells you what the graph is about.
• Category axis: lists the groups being compared (often the horizontal axis).
• Value axis: shows the numerical scale, starting at 0 with equal intervals.
• Bars: rectangles whose length represents the value for each category.
• Legend: used when a graph compares more than one data set, such as in a stacked bar graph.

To read a bar graph accurately, follow these steps:

1. Read the title to understand what data is displayed.
2. Check the value axis to find the scale and interval between grid lines.
3. For each bar, trace from the top of the bar to the value axis to find its value.
4. Compare bar lengths to see which category has the greatest or least value.

In the fruit graph above, each grid line represents 5 votes. The banana bar reaches the line at 15, so 15 people chose banana. The grape bar is the tallest, so grape received the most votes overall.

Drawing a bar graph from a data set follows a similar logic in reverse:

1. List the categories and their values in a table first.
2. Draw and label the category axis and the value axis.
3. Choose a scale for the value axis that comfortably fits your largest value, using equal intervals starting at 0.
4. Draw each bar with the same width, spaced evenly, rising to the height that matches its value.
5. Add a title and label both axes, including units where needed.

For example, suppose 4 classes recorded the number of books read this month: Class A read 12, Class B read 18, Class C read 9, and Class D read 15. If you choose a scale of 3 per grid line, Class A's bar would rise to the fourth line, Class B's bar to the sixth line, and so on. Keeping every bar the same width keeps the comparison fair and easy to read.

A stacked bar graph splits each bar into segments to show how a total is divided among sub-categories. To read one, look at the full height of the bar for the overall total, then look at each colored segment's length for the value of that sub-category. You can find one segment's value by subtracting the running total below it from the running total at its top.

In this stacked bar graph, Year 1's total height reaches 30, made of Toys (10), Books (10), and Games (10). Year 2's total reaches 40, with Toys at 20, Books at 10, and Games at 10. Comparing the full bar heights tells you the overall total grew from Year 1 to Year 2, while comparing individual segments shows exactly which category grew.

• Always start the value axis at 0 so bar lengths stay proportional and comparisons stay honest.
• Keep bars evenly spaced and the same width; only their length should change.
• Choose a scale interval that divides your data range evenly, like counting by 2s, 5s, or 10s.
• Double-check units in the title or axis label before drawing conclusions from the graph.

Bar graphs work best for comparing separate categories. If your data changes continuously over time, such as temperature each hour, a line graph may show the trend more clearly. If you are choosing between different chart types for a data set, it helps to review the advantages and disadvantages of different graphs. When your categories involve continuous numerical ranges instead of separate groups, look at frequency distribution and histograms instead.

Related lessons

• Statistics Help