A Little Something About Pie Charts

Everyone knows about pie charts so I won’t bore you with an introduction. What I will say, is that pie charts are fascinating circles of controversies; users love them and experts like Stephen Few, despise them. Even the Export notes section of IBM Cognos says the following:

“Pie charts have a mixed reputation. They are popular in business and the media but many information designers have criticized the technique. Some claim that the pie slice shape communicates numbers less exactly than other possibilities such as line length.”

So, what’s with all the pie chart hate? Experts time and time again avoid the little circles that could (or couldn’t), because:

  • It is hard to tell the exact value of a pie slice without using data labels, and
  • One cannot draw a precise comparison between two slices in a pie chart.

(You can see an example of pie chart misuse in our post Circle Circle Dot Dot – What some dashboards should have not.)

The real problem with pie charts isn’t that they simply suck, it’s that most people aren’t using them correctly. Ian Spence, Professor in the Department of Psychology at the University of Toronto, pointed out in his article No Humble Pie: The Origins and Usage of a Statistical Chart that:

“The pie chart is a simple information graphic whose principal purpose is to show the relationship of a part to the whole. It is, by and large, the wrong choice as an exploratory device, and it is certainly not the correct choice when the graph maker or graph reader has a complicated purpose in mind, such as displaying small changes in proportion over time, a task that would require several pies.”

Suppose you run into a situation where:

  • You need to create a data grid jam-packed with data labels.
  • You need to represent part-of-whole relationship between two columns.
  • You have very limited horizontal screen estate.
  • Each row of data should be read separately. In other words, you do not want to encourage comparison between two rows.

Think about having pie charts that are displayed within the data grid, like the following:

Pie charts are not completely unnecessary, and they have been proven to be useful in some cases. We should still remain skeptical about their usage, but I don’t believe it’s time just yet to throw them out completely.

About the Author

Written by:

Edmond is a Business Intelligence Consultant at Dundas Data Visualization, Inc. who has delivered dashboard projects to many different industry sectors. He believes the building of a dashboard is both an art and a science.

Comments (4)

  1. Submitted by jesus ponce

    thanks, for your information. I like to use the pies along with bar graphs.
    The pie show only the total of the items and the bar graphs aside from the pie, showing the specific values of each slice of the pie with a mouse over effect.

    saludos.

    Reply
  2. Submitted by Wayne G. Fischer, PhD

    o Better check your pies…”Linen Supply” slice…

    o “…you do not want to encourage comparison (between two rows).” The main purpose of data visualizations *is* to encourage comparisons! And even in your example, it would be very natural (and useful in a business sense) to compare the various shipping cost proportions.

    o “…displaying small changes in proportion over time, a task that would require several pies.” OMG, first you tell why a pie chart is almost always a poor choice for comparisons, then quote someone who recommends using them in series, to display proportions over time. Absolutely not! Use the SPC p-chart. Sheesh…

    Reply
  3. Submitted by Edmond Chan

    Wayne, my goal is to demonstrate the good and bad of pie charts at the same time. The pie charts encourage column comparison (Item Cost and Shipping Cost) over row comparison.

    The quote from Spence supports my argument that pie charts are only useful in certain circumstances. They are designed for showing part-to-whole relationships instead of time series relationships.

    Reply

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