Double Line Graph: What It Is, Examples and How to Read One

A double line graph has two lines on the same grid. Each line shows one set of data, and because they share the axes, you can see at once which is higher, how far apart they are, and whether they rise and fall together. It is one of the first chart types students meet in school, and it is just as useful at work. This guide explains what a double line graph is, walks through examples, shows how to read the gaps and crossings, and covers how to plot one and what can go wrong.

What a Double Line Graph Is

Start with the single line graph. A line chart displays a series of data points, called markers, connected by straight line segments. The points are ordered, usually by their x value, and it is often used to show a trend over time, according to Wikipedia. Claus Wilke’s open textbook Fundamentals of Data Visualization describes the same idea: in a time series each dot has a defined order, and connecting neighboring points with lines emphasizes that order.

A double line graph simply puts two of these lines on one grid. The two lines share the same horizontal axis (often time) and the same vertical scale. That shared scale is the whole point: it lets you compare.

Where the Name Comes From

“Double line graph” is the term used in schools. We did not find it defined in a textbook or a standards document, so treat it as a common name and not a formal one. Wilke’s chapter on the topic is titled multiple time series. Whatever you call it, the picture is the same: more than one line, one set of axes.

Example: Two Cities’ Temperatures

Here is a double line graph of the average daily temperature in two cities over a year. The numbers are sample data.

Notice how much you can read without a single number:

• Both lines climb from January to July, then fall. The seasons shape both cities.
• City B is above City A in every month. It is the warmer city all year.
• The gap between the lines is wide in winter and narrow in summer. In January City B is 14 degrees warmer, and in June and July only 2 degrees warmer.

A table of 24 numbers would hold the same information, but the shape of the gap is much harder to see in a table.

Open this double line graph in the line graph maker

How to Read a Double Line Graph

Read it in this order and you will rarely go wrong.

1. Read the title and both axis labels. What is measured, and in what unit? Check the scale on the vertical axis too.
2. Match each line to its color in the legend. Do this before looking at any shape.
3. Follow each line on its own. Is it rising, falling or flat? Where is the highest point and where is the lowest?
4. Compare the two lines. Which one is higher? By how much? Does the gap widen or narrow?
5. Look for crossings. A crossing is a point where the lines swap places, so the leader changes.
6. Check whether the comparison is fair. Do both lines use the same unit and the same scale?

Reading the Gap Between the Lines

The vertical distance between the lines at any point is the difference between the two values there. A gap that is growing means one series is pulling away. A gap that is shrinking means they are converging. Parallel lines mean the difference stays the same while both series move.

Reading a Crossing

A crossing means the leader changed. Here is a cafe whose iced drinks and hot chocolate sales trade places twice a year.

Two things to say about a crossing. First, the crossing usually falls between two data points, so you can tell in which interval it happened but not the exact day. Second, a crossing is not the same as both values being equal at a measured point. Use the data table if you need an exact answer.

Open the cafe graph in the line graph maker

Practice Questions

Try these on the two-city graph above, then check the answers against the data table under the chart.

1. In which month is the gap between the cities largest? January and February, where City B is 14 degrees warmer.
2. In which month is the gap smallest? June and July, where the gap is 2 degrees.
3. What is the difference in April? City A is 59 and City B is 66, so the difference is 7 degrees.
4. Do the lines ever cross? No. City B is warmer in all twelve months.
5. Which city has the bigger swing across the year? City A. It goes from 38 to 82, a range of 44 degrees, while City B goes from 52 to 84, a range of 32.

Question 5 needs a second look at the picture: City A’s line is steeper, because it covers more degrees between winter and summer.

When to Use a Double Line Graph

Use one when you have two measurements of the same kind, on the same scale, over an ordered variable such as time. Good uses are two classes’ test scores across a school year, sales this year and last year by month, and the temperatures of two cities. Skip it when the two series use different units, when the horizontal axis holds unordered categories, or when you have so many points that the lines tangle.

How to Plot a Double Line Graph by Hand

You need graph paper, a ruler and two colored pencils. The steps are the same whether you draw the graph by hand or in a tool.

1. Choose the axes. Put the ordered variable, such as time, on the horizontal axis. Put the measurement on the vertical axis.
2. Pick one scale that fits both sets of data. Look at the smallest and the largest value across both series, and let the vertical axis run a little beyond each end. Number the axis at equal steps.
3. Plot the first series. Mark a dot for each data point and connect neighboring dots with straight segments, in one color.
4. Plot the second series. Use the same axes, a different color and, if you like, a different dot shape.
5. Add a title, axis labels and a legend or direct labels. Say what each color means.
6. Check it. Does every line have one point per category? Are both lines in the right colors?

Wilke adds a design tip for graphs with several lines: a separate legend creates cognitive load, and labeling the lines directly reduces it. On paper that is easy: write the name next to the end of each line. Our line graph maker uses a legend above the chart, so for two lines, make sure the two colors are easy to tell apart.

Three or More Lines: The Multiple Line Graph

You can add a third line, and a fourth, on the same grid. Wilke’s textbook covers this case: with many time courses on one chart, connecting the dots with lines helps the viewer follow each course. Here are three sources of website visits.

Three lines still read well here because they rarely cross. Each additional line raises the chance of crossings and tangles. Here is our own rule of thumb, an editorial judgment and not a finding from a study: if you cannot follow each line in a few seconds, split the chart in two.

Common Mistakes With Double Line Graphs

Two Different Units on One Axis

The most common problem is putting two things on one scale that do not belong together. Here are temperature in degrees Fahrenheit and rainfall in inches on a single axis.

The rainfall line looks almost flat, yet rainfall varied from 2.8 to 4.2 inches. The scale, built for temperatures, hides it. Our line graph maker has one vertical axis, so it cannot draw a second scale on the right side. That is a limit of the tool, and it also points to the simplest fix: give each measurement its own chart.

If you need to compare two units, put two small charts one above the other and line up their months.

Too Many Lines

Past three or four lines, most charts start to tangle, though it depends on how often the lines cross. That is our rule of thumb, as above, not a published limit. Split the chart, or highlight one line and keep the rest gray.

Lines Between Points That Are Not Ordered

A line implies order and change between points. Wilke notes that line graphs are appropriate whenever the data points have a natural order along the x axis. For plain categories, such as ice cream flavors, use bars instead.

A Truncated Axis Without a Note

Our line graph maker has a switch called Start axis at zero. We did not find a textbook rule that says line graphs must start at zero, and in a line graph the slope carries the message. But if you zoom in, the same data looks more dramatic. Say what range you used, or turn on the zero baseline when the size of the values matters.

Missing Values

If a value is missing, leave the cell empty. The line breaks at that point, and the reader sees that the data is missing instead of a fake drop to zero.

Double Line Graph vs Double Bar Graph

A double bar graph shows two bars side by side for each category. Wikipedia calls this a grouped, or clustered, bar chart, where each group has two or more bars color-coded for a particular grouping. The same temperature data can be drawn either way.

Which one should you choose?

• Use a double line graph when the horizontal axis is ordered, such as time, and the point is the trend or the gap.
• Use a double bar graph when the categories are separate things, such as classes or products, or when you have just a few of them and the exact values matter.

If your data has both, such as sales by product over many months, the lines are usually clearer than 12 groups of bars. For more chart types and a chooser table, see our guide to types of graphs and charts.

Start From a Free Template

To draw your own double line graph, open the template below. It has two series, five rows and placeholder names. Type over them, or paste a table from a spreadsheet with the labels in the first column and a header row.

Open the double line graph template

The line graph maker draws each column as one line, lets you smooth the lines, add markers, fill under the lines, and download a PNG or SVG or copy a link to your chart. It runs in your browser, with no sign-up.

A Short History Note

Stephen Few writes that William Playfair did not invent the line graph but popularized it as a means to display quantitative values across time. Wikipedia lists Playfair among the earliest people to use it. A double line graph applies that same old idea twice on one grid.

Summary

A double line graph has two lines on one set of axes so you can compare them. Read each line first, then the gap, then any crossings. Keep both lines on one scale and in one unit, keep the number of lines small, and use bars when your categories have no order. Then plot your own in the line graph maker.