Stack Plots

1. Stack Bar Graphs

Stack plots are plots that stack one on top of each other so that you can see how they relate to the totals. Stack plots allow you to break down bar graphs or line charts into component pieces to see the components more clearly. The problem with stack plots is that sometimes they can be hard to interpret.

EXAMPLE

Suppose that a company spent a certain number of millions of dollars over these 5-year intervals, as shown in the graph below. So, in 1985, they spent $10 million. In 1990, they spent a little bit less, and so forth. It looks like their spending increased significantly in the last 10 years or so.

What a stack plot will do is allow us to see exactly where that spending breakdown occurred. Look at the stack bar graph below, which breaks out each measured year's spending into its components:

• Operations, which is the day-to-day business of the company
• Marketing, which is the promotion side of the business

If you look at the stack bar graph, you can see that operations (the green component) have grown a little bit over the years that the company has been in business, perhaps due to inflation. The green bars stay about the same height throughout this period of time.

The marketing budget in yellow, however, has really proliferated over that same period of time.

think about it

If the company is looking to cut costs, do you think they would cut from the operations budget or the marketing budget? Well, perhaps they'll cut from marketing because they can see that by the year 2010, marketing and operations are costing about the same amount.

You can also see that the marketing budget didn't increase or decrease significantly within the 5 years from 2005 to 2010, as the size of the yellow ribbons is approximately the same size in those 2 years. This is simply another way to use this type of breakdown to analyze the data.

term to know

Stack Plot

A bar graph or line chart that is subdivided into its components so that the comparisons, as well as the totals, can be seen.

2. Stack Line Charts

A stack line chart can be problematic because they are often very hard to read. It is difficult to tell whether the values are cumulative or individual.

EXAMPLE

Suppose there are four different boutique stores in different locations that a company owns. They want to know how the business is doing in each of these four markets.

Based on this graph, it's hard to tell whether the New York store is doing poorly because it's at the bottom, or whether these are stacked on top of each other. If they are stacked, then the Miami store is doing the worst because its difference is the least from the next lowest one. Does the Miami store make $250,000 on its own? Or is it a total of $250,000 between the four stores and each of these are components?

It's hard to tell whether these are added values or individual values. You can make it clearer to interpret by making the lines ribbons instead, as shown below.

Now you can see what revenue each store accounted for, in each measured month. By using ribbons instead of lines, it is clearer that you're talking about adding the values, as opposed to individual values.

term to know

Stack Line Chart

A line chart where the lines represent cumulative amounts rather than individual amounts. These are typically done with different colored "ribbons" to make it clearer that we are talking about totals.