Pie graphs can be great at showing relative differences between a small number of categories. A recent example of this comes from CBS:
CBS This Morning co-host Tony Dokoupil set up a table at a mall in West Nyack, New York, with a pie that represented $98 trillion of household wealth in the United States. The pie was sliced into 10 pieces and Dokoupil asked people to divide up those pieces onto five plates representing the poorest, the lower middle class, middle class, upper middle class, and wealthiest Americans. No one got it right. And, in fact, no one was even kind of close to estimating the real ratio, which involves giving nine pieces to the top 20 percent of Americans while the upper middle class and the middle class share one piece between the two of them. The lower middle class would effectively get crumbs considering they only have 0.3 percent of the pie. What about the poorest Americans? They wouldn’t get any pie at all, and in fact would get a bill, considering they are, on average, around $6,000 in debt…
To illustrate just how concentrated wealth is in the country, Dokoupil went on to note that if just the top 1 percent are taken into account, they would get four of the nine pieces of pie that go to the wealthiest Americans.
A pie chart sounds like a great device for this situation because of several features of the data and the presentation:
1. There are five categories of social class. Not too many for a pie chart.
2. One of those categories, the top 20 of Americans, clearly has a bigger portion of the pie than the other groups. A pie chart is well-suited to show one dominant category compared to the others.
3. Visitors to a shopping mall can easily understand a pie chart. They understand how it works and what it says (particularly with #1 and #2 above).
Together, a pie chart works in ways that other graphs and charts would not.
(Side note: it is hard to know whether the use of food in the pie chart helped or hurt the presentation. Do people work better with data when feeling hungry?)