Line Chart

In this tutorial, you're going to learn about line charts. This will cover:

1. Line Charts
2. Frequency Polygon
3. Multiple Line Charts

1. Line Charts

Line charts are a kind of display that can be used quantitative data, using a construction that is very similar to a histogram (refer to the tutorial on how to construct histograms for more about those graphs).

Line Chart

A distribution of quantitative data that shows the frequency of different intervals of data. The frequencies are indicated by heights of dots, which are connected to each other.

Suppose that we have an elementary school class in Muncie, Indiana, say, keeping track of the high temperature for each school day. In Indiana, it might get down to, say, 0℉ in the winter, or as high as 90℉ at the beginning of the school year in September or at the end of the year in May. That distribution could look something like this:

The histogram for this distribution looks like this.

There were 10 days where the temperature fell between 0℉ and 10℉, 16 days that were between 10℉ and 20℉, etc..

So how do we transform the data in this histogram into a line chart?

To create a line chart, take the heights of the bars, and instead of creating them as heights of bars, create dots instead.

Then get rid of the boxes and connect the dots instead.

This is a line graph, and it's almost the same visual display is the histogram showed.

As with histograms, binning makes a difference. If you bin differently, such as by fives instead of by tens, you end up with a graph that looks this:

2. Frequency Polygon

Finally, if you put the histogram and the line chart on the same set of axes, you can create something called a frequency polygon.

Frequency Polygon

A distribution of data that shows both a histogram and its line chart on the same set of axes.

When you have the histogram and the line chart together, you can connect the tops of the midpoints. That would look like this:

3. Multiple Line Charts

It's also possible to do multiple line charts on the same set of axes.

Multiple Line Charts

A distribution that shows more than one data set's values in line charts. This is advantageous because it is clearer than trying to compare multiple histograms on the same set of axes.

This is one way that line charts are helpful, because it's not really all that possible to plot multiple sets of data with a histogram.

Suppose that a school in Tucson, Arizona did the same project as the kids in Muncie, Indiana did, and then the two schools shared their information.

The line charts, then, can be compared, showing the similarities and differences between the two data sets. Days in the 60℉s, 70℉s, 80℉s, 90℉s, and even 100℉s are very common in Arizona. Whereas in Indiana, the days that are most common are days in the 30℉s, and 40℉s, and 50℉s.

Line charts are nice way to visualize quantitative data. They use much the same construction as a histogram, but the heights are determined by dots and set of boxes; when both a line chart and a histogram are shown on the same set of axes, you can see the frequency polygon. The best use of a line chart is when you can compare a couple of different line charts on the same set of axes, creating multiple line charts.

Thank you and good luck!