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What Are Line Charts?

You use a line chart when you want to present data over time. The table and line chart below shows temperatures that were measured one day in May between 02:00 and 12:00.

Table of temperatures at different times

Line chart of temperatures at different times

Look at the line chart. The six points in the table are drawn in the coordinate system above, and lines are drawn between the points.

In this line chart, the time is plotted along the first axis (the horizontal), and the temperature along the second axis (the vertical). Each point in the line diagram represents a time with the corresponding temperature. When you look at a point, you can go vertically down and read the time, and horizontally to the left to read the temperature. For example, you see that the warmest temperature is 18 °C and it was 8 pm. The coldest temperature is 5 °C and it was measured at the time 02:00. In general, we use a line chart when we look at a change that happens over time.

Think About This

What if we had measured the temperatures in December instead of in May? What would the table and bar chart look like then?

Table of temperatures at different times

Line chart of temperatures at different times

From the line chart you can now see that several of the values are below the first axis (the horizontal axis). This is because the temperature is a negative number. In cases where the observation is negative, the diagram will be below the first axis.

The information in the table is drawn in the line chart below. The table shows an overview of the number of customers and times:

Table of number of customers at different times

The table shows the number of customers who stopped by a store between 6 am and 6 pm. They are registered so that customers who came between 6 and 8 am, is listed at 8 am. Customers who came between 8 and 10 am, was logged in at 10 am, and so on.

Line chart of number of customers at different times

The line chart is created by drawing the points from the table in the coordinate system above, and draws lines between all the points.

Could this chart be drawn in some other way to change the perceived information?

Think About This

Study the line chart below. What do you see?

Line chart of number of customers at different times

This line chart is the same as you saw above, but the axes have been changed. In this plot, the differences in number of customers at the different times looks smaller. With small changes, information can be displayed very differently (manipulated) to present the picture you want. With statistics, it is often the case that if the publisher benefits from the differences in number of customers being small, they will press the second axis together. While if the publisher wants to make it clear that the discrepancy is large, they pull the second axis so that it becomes longer. Such manipulation is not good, but is unfortunately very widespread. So stay alert!

It is your responsibility to notice if information is being manipulated. It is therefore important that you look closely at how the information is presented, so you are not fooled! It is your responsibility to check sources and analyze charts when reading them. In order to fully understand the information you are given, you must have good math skills. Percentage calculation and basic arithmetic take you quite far.

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