What Are Line Charts?
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You use a line chart when you want to present data over time. The table and line chart below shows temperatures that were measured during one day in May between 02:00 and 12:00.
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 chart represents a time with a 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 was °C at 8 am. The coldest temperature was °C and it was measured at 2 am. 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?
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.
This table shows the number of customers who stopped by a store between 6 am and 6 pm. The data is organized so that customers who came between 6 and 8 am are listed in 8 am slot; customers who came between 8 and 10 am are logged in at 10 am, and so on.
The information in this table is drawn in the line chart below.
The line chart is created by plotting the points from the table in the coordinate system above, and drawing lines between all the points.
Could this chart be drawn in some other way to change the way the data is perceived?
Think About This
Study the line chart below. What do you see?
This line chart has the same data in it as the one you saw above, but the axes have been changed. The way this chart is plotted, the differences between numbers of customers at different times looks smaller. With small changes, information can be displayed very differently—manipulated—to present the picture you want.
Using this example, it is often the case that if, say, the publisher of this chart would benefit from the differences in customers from hour to hour being relatively small, then they will compress the second axis to make the differences appear lessened. Whereas if the publisher wants to make it seem that the discrepancy is large, they would stretch the second axis so that it becomes longer. Such manipulation is not good, but it 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’s necessary for you 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 calculations and basic arithmetic will take you quite far, in addition to thinking critically about the things that are presented to you.
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