What is a Line Graph? — Definition & Examples
What is a grid?
The grid that forms the foundation for a line graph has two axes, the x-axis running horizontally and the y-axis running vertically. These two axes need not be in the same scale, meaning their increments need not match. This is because the x-axis represents one category of information related to time, like years, months, days, or hours. The y-axis represents a set of numbers we are examining over that span of time.
The word "graph" comes from Greek, meaning "writing," as with words like autograph and polygraph. A line graph uses a grid of intersecting perpendicular lines formed by an x-axis and a y-axis, over which you show changes over time, using a line. The most important feature setting a line graph apart from, say, a bar chart is the graphical way it shows the passage of time.
The grid can be shaped in three ways: precise, tiny squares; tall, narrow rectangles; or wide, short rectangles. Sometimes line graphs deliberately exaggerate one axis to emphasize the information.
Here are two line graphs of the same information, but in the second one, we have doubled the height of the vertical increments and halved the width of the horizontal increments, making the information look much different:
The second graph would make many people very nervous that the prices being tracked are out of control, rising and falling dramatically. Yet the two graphs show the same information.
What is a line graph?
A line graph is a visual representation of a single piece of information tracked over time. You can see line graphs with multiple lines, but each line tracks one idea and is moving through time from earlier time (on the left) to more current time (on the right).
A line graph has a title, nearly always centered at the top and above the graph. The title explains what the two axes are showing: "Gold Prices Per Ounce 1998-2018," or "Hank Aaron's Career Home Runs Per Season," or "2018 Temperatures in Tempe, AZ"
A line graph also has axis labels, with the y-axis label usually printed sideways and running along the y-axis. This will be the quantity, like "Price in Dollars," "Number of Home Runs," or "Temperature in ℃."
The x-axis label is a unit of time written horizontally under the numbers, ranging from centuries to seconds as you need. In our three examples, the x-axis might be labelled "Year" for the first two graphs, and "Weeks" or "Months" for the last one.
Increments for the y-axis are determined by the range of data. If gold prices from 1998 to 2018 varied from a low of $272.65 (in 2000) to a high of $1,664 (in 2012), we do not need to start at $0 and climb by $10 increments. The graph would be absurdly tall. We could start at $200 and move up by $100 increments, ending at $1,700.
Increments for the x-axis are determined by the range of time, with common units of years, months, weeks and days.
A key tells what the changing line shows, like "Price," "Home Runs," or "Temperature."
Example of a line graph
Data displayed by a line on a line graph is great for spotting trends, noting consistent performance, or tracking progress. You could make your own line graph of mathematics quiz grades over a term. Your x-axis would be days or weeks, and your y-axis would be your grades.
Give your line graph a title.
Use subtraction to find the range of your data, so your y-axis does not waste white space with grades from 0% to 50%, but instead starts at a grade that is just below your lowest grade.
Determine the increments for your two axes, like five or 10 percentage points on the y-axis and weeks on the x-axis.
Label both axes.
Chart your data points at each intersection of the x-axis and y-axis.
Write in a key explaining your line.
Here is a sample line graph for Sally Peprally:
Sally charted her progress to help her decide if she should enroll in the next higher mathematics class for the next term. She can look at the line graph and get answers to questions that range from simple to complex. See if you can answer them:
What is displayed?
Which way is the line trending?
Which two weeks were the same?
Does the line graph indicate any problems?
Should Sally Peprally attempt the next higher mathematics class?
Did you get these answers?
Sally's mathematics quiz grades are displayed.
The line is trending upwards.
Week 2 and Week 5 had the same grade.
The line graph showed Sally's difficulty in Week 5.
Sally should consider the next mathematics class, because she is steadily improving.
Lesson summary
Now that you have moved through the whole lesson, you are able to recall and state the definition of a line graph, identify the features of a line graph, including the x-axis, y-axis, titles, labels, and line for discrete data points, construct your own line graph, and interpret results recorded on an existing line graph.