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Updated on Aug 25, 2011

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LESSON SUMMARY

Writers can use graphs and charts to communicate information. In this lesson you learn how to interpret three types of graphs: bar graphs, line graphs, and pie graphs.

Have you ever noticed that your math and science textbooks include a lot more graphs and charts than your language arts textbooks? Graphs can be a very useful tool for conveying information, especially numbers, percentages, and other data. Scientists and mathematicians often use graphs to show the results of studies or experiments. A graph gives the reader a picture to interpret. That can be a lot more efficient (and interesting!) than writing pages and pages to explain the data.

Graphs can seem intimidating, but reading a graph is a lot like reading a story. The graph has a title, a main idea, and supporting details. You can use your active reading skills to analyze and understand graphs just like any other text. First, let's look at the parts of a graph.

### The Parts of a Graph

Most graphs have a few basic parts:

• a caption or introduction paragraph
• a title
• a legend or key
• labeled axes

An active reader looks at each part of the graph before trying to interpret the data.

### Caption or Introduction

Before you even look at the graph, you'll want to look for clues to its significance. If the graph is shown in the middle of a paragraph, read the text before it. It will probably introduce the topic and suggest the author's purpose in including a graph. What point is the author trying to illustrate? Is the graph being used to prove the writer's point or strengthen an argument?

If the graph is off to the side of the text, look for a caption underneath. Captions will usually tell you where the data came from (for example, a scientific study of 400 African elephants from 1980 to 2005). Captions usually summarize the author's main point as well, which is the reason for including the graph.

### Title

The title is very important. It tells you the main idea of the graph by stating what kind of information is being shown. A graph might have the title, "NASA program spending from 2002 to 2008." You can already guess that the graph will show some different NASA programs, some years, and some monetary amounts. Just by reading the title, you can start to make predictions about the data and prepare your brain to interpret the graph.

### Legend

A legend, also called a key, is a guide to the symbols and colors used in the graph. It tells you what is being measured on the graph. You can't make sense of the graph without first looking at the legend. Here are some examples:

Not every graph will require a legend, but if the graph shows multiple colors, solid and dotted lines, or symbols or icons, be sure to look for the legend. Then you'll understand what data are shown and how the data are organized.

### Labeled Axes

Many graphs, including bar graphs and line graphs, have two axes that form a corner. Usually these axes are the left side and the bottom of the graph. Each axis will always have a label. The label tells you what each axis measures. Look at these examples:

In the first example, the bottom axis, called the x-axis, shows time in months. The vertical axis, or the y-axis, shows the amount of money spent. According to the text in parentheses, the y-axis is measured in dollars. Thus, dollars are the unit of measurement for this graph. The second example uses years as the unit of measurement for one axis, and millions of people for the other axis. Notice the word millions in parentheses. The graph might seem to show about "300 people" in the year 2008, but the units tell you to read it as "300 million people."

The units on each axis won't always start at 0, but they will follow a consistent interval. For example, a graph that measures the U.S. population might start at 50 million if there are always at least 50 million people in the data. A graph that tracks monthly expenses might show 5, 10, 15, 20, 25 on the y-axis, an interval of \$5. This makes the graph easier to read, and you can still plot other numbers, like 18, by placing them in the correct spot, in this case about halfway between 15 and 20.

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