A Bar-of-Pie Chart. And a Pie-of-Pie Chart may be even worse. Let the Chart Busters explain.
Last week, Excel guru John Walkenbach conducted a poll on his wildly popular J-Walk Blog asking people to Post Your Pet. The poll results were tabulated in Tabulating The Pets.
I know John’s an expert and all, and he literally wrote the book on Excel Charts. Make that, the books. And these are good books, starting with the basics and progressing into some intermediate topics.
I know the books are good, because I helped out with technical editing. I made sure the protocols and the tricks all worked, and I checked out all the sample files. I even encouraged discussions of proper chart type selection.
Disclosure: these book covers link to Amazon.com, and if you buy one after following my links, I get a commission of a few percent.
Something-of-Pie Charts
So how did John decide to present the results?
Besides the usual yuck of a pie chart, the stacked bars in the bar-of-pie chart provide even more distortion. See the bar for Guinea Pig? If the labels weren’t there, I would have thought there were more guinea pigs than hamsters, rats, and other, combined. This is a problem with encoding values in areas, particularly when the scale of area to unit changes so drastically.
By default, Excel makes the bar 75% as tall as the pie’s diameter, or 37% of the pie’s area. The label on the “other” slice of the pie tells me that the bars comprise 5 units, or 5.4% of the total, but my visual cortex is overwhelmed by the hugeness of the bars.
How about a pie-of-pie chart?
After getting over your impression of a wagon with mismatched wheels, you are misled even worse by this graph. The default “other” pie has 75% the diameter and 56% the area of the main pie. It’s obvious that guinea pigs is as large a wedge as cats, right?
Can you say “cognitive dissonance”? Sure you can. As Professor Tufte has said, the only thing worse than a pie chart is more than one of them.
Improved Chart Selection
So how do you show all elements in a single chart with a single encoding scale? Class?
That’s right, our old boring friend, the bar chart. Look, guinea pigs are nowhere near as popular as cats after all.
What if you want to show the actual counts in the chart?
Easy. Data labels add the information, without being critical to your understanding of the chart.
Naomi B. Robbins says
Good points, Jon. I highly recommend John Walkenbach’s book but dislike his treatment of histograms.
John Walkenbach says
My plan worked! When I created that chart, I knew it was not the best way to present the data, but I did it anyway. I was actually hoping that it would get some attention from the PTS Blog.
If I were presenting this data for a serious audience, I would have used a bar chart.
Naomi… if you have some time, please send me an email with your thoughts on histograms. I’m revising the books right now and this is a good time make changes.
Naomi B. Robbins says
To quote Leland Wilkinson in The Grammar of Graphics (page 2) “A histogram is not a bar chart.” To start, a bar chart has a categorical horizontal axis while a histogram has a binned quantitative axis. Therefore, there should be no spaces between the bars. I have never seen a pseudo- third dimension in a histogram other than in Excel Charts. I’ll send more details in an email after I meet some deadlines. If you are in a hurry I suggest looking at the Grammar of Graphics.
Jon Peltier says
From “The Grammar of Graphics”, earlier in the same paragraph that Naomi cites:
This book shuns chart typologies.
Wow, that’s good. I’ll have to get a copy.
John Walkenbach says
Thanks for setting me straight, Naomi. I didn’t realize the histograms don’t have gaps between the columns. I did a Google image search for ‘histogram’ and just about every example had no gaps. The exceptions are the histograms generated by the Analysis Toolpak add-in.
I’ll fix those histograms in the 2007 editions. For now, I’ll just blame my tech editor for not catching it. :)
jeff weir says
Jon: My cat ate my mouse, so I couldn’t vote in the original poll. Can you amend the above graphs accordingly.
Jon Peltier says
Jeff – You’re barking up the wrong tree.
Vishal says
Thanks a lot.
Depending on the message to be given, in the bar chart, I may put “Others” in a single bar and split that bar by cluster. Helps when individual data is too small to be noticed otherwise and also the other bars are scaled appropriately.