What is an Area Stack Chart?
An area stack chart is a data visualization which depicts a quantitative variable (on the y-axis) over time (on the x-axis). The variable on the y-axis is broken down into multiple series, or groups, which are stacked on top of each other with the area in between them representing the value of the specific series. The area stack chart is a variation of a line chart with multiple series, where the area under each line is filled in and stacked on top of one another.
When to Use an Area Stack Chart
Area stack charts are used to display the portion that each series contributes to the total. This is where area stack charts differ from multiple line charts. With area stack charts, it is easy to distinguish the total sum of the series and relatively how much of that total is made up of each series. Area stack charts are best used when there are a small amount of groups on the chart. By having only a few groups, it is much easier to determine the total of each series and how each series contributes to the total.
In the example above, the area stack chart depicts the UFO sightings in the USA by year. Each year is separated into categories based on the shape of the UFO. By having only three different categories, it is easy to not only determine the total count of sightings by year, but also to visualize the contribution of each category to the total.
When NOT to Use an Area Stack Chart
By including a large number of categories, an area stack chart can become messy and difficult to read.
In this area stack chart, we are again looking at UFO sightings by year, but also include a larger number of UFO shapes. While we can determine the total number of sightings by each year, it is impossible to distinguish the individual series. Also, there are so many series that there are not enough colors for each series to have its own color. This makes it even more difficult to determine which series is which.
Comparison of Like Chart Types
Another similar chart type that can be used to visualize individual contributions of parts to a whole is column stack chart. A column stack chart, like the area stack chart, visualizes a quantitative variable on the y-axis over time on the x-axis. The quantitative variable is also split into categories that are stacked on top of each other. Both charts are used to not only show a total value but, more importantly, break down that total into its individual contributions. The difference between the two is that a column stack chart uses individual columns for each time period which are not connected. Below is a table which compares the uses of each of these types of charts.
Area Stack Chart
Column Stack Chart