This blog entry was written by Bryan Pierce of Perceptual Edge.
Here at Perceptual Edge, we like to show real-world examples of poor graph design to teach people what not to do, because knowing what to avoid and why it doesn’t work is an important step in the learning process. We often receive emails from people who follow this blog or have read Stephen’s books or articles who want to share examples that they’ve come across. The pie chart below is one such example:
This graph was produced by the Global BusinessObjects Network to promote the upcoming 2009 SAP BusinessObjects User Conference. It’s supposed to show what BusinessObjects products the attendees of last year’s conference used. Regular readers of Stephen’s work know that he dislikes pie charts because they don’t work as effectively as alternatives like bar graphs, so I won’t revisit the general problems with pie charts. (If you’re interested in more information about pie charts, Stephen wrote a full review detailing their significant problems and single, rarely-needed strength.) Unfortunately, the design of this graph is quite terrible, even by pie chart standards.
This graph is dysfunctional for two major reasons. First, only the large slices have been directly labeled. All of the small slices are labeled using a legend, but there are so many slices that it’s impossible to associate the colors in the legend with the slices in the pie because so many of the colors are so similar. Sure, people can just read the values from the legend and ignore the pie, but how is that better than a simple table?
The second major problem with this graph is this: Pie charts are designed to display part-to-whole data, with each slice representing one discrete part of the whole and all of the slices adding up to 100%. For instance, you could show the breakdown of sales by region for a company. In the graph above, however, the slices add up to significantly more than 100% because the categories aren’t mutually exclusive. For instance, 67% of attendees use BusinessObjects Web Intelligence, but many of those people use other BusinessObjects software, too, so they’re being counted several times. The end result is that the blue slice that represents BusinessObjects Web Intelligence has a value of 67%, but it only takes up about 15% of the space in the pie. The visual picture conveyed by the pie chart misrepresents the data.
Both of these problems could have been solved by using a horizontal bar graph. With a horizontal bar graph, all of the bars could be the same color and there would be no problem labeling all of the slices directly, which would address the first problem, and because bar graphs are more versatile than pie charts and can be used for more than just part-to-whole relationships, it wouldn’t be confusing when the bars added up to more than 100%, which solves the second problem.
At Perceptual Edge, we’ve seen plenty of graphs like this, and worse. But it always irks us when we see examples like this coming out of the Business Intelligence industry, an industry that should know better. In this case, the Global Business Objects Network puts on a large conference with dozens of educational courses on BI related subjects, including analytics and dashboards. How do they expect people to trust them with the sophisticated visualization training, when their simple graphs are so dysfunctional?