Pie Charts

Chandoo says we can Group Smaller Slices in Pie Charts to Improve Readability. Such a pie has too many labels to fit into a tight space, so you need to move the labels around and use leader lines to link the labels to their data points.

An extreme example? Judging from what I see around the web, this is not at all unusual. In fact, 12 points isn’t all that many for a lot of pie charts. They could use a readability improvement.

Chandoo’s technique is to convert the chart to a Pie-of-Pie type, which lets you move some slices to a second pie, leaving a bland “Other” wedge in their place. This can be a deceptive chart. Not only don’t two wrongs make a right, the secondary pie is 75% of the diameter and 56% of the area of the main pie, while having an actual value of only 5% of the main pie.

Chandoo cleans up the chart by moving most of the slices to the secondary pie, then shrinking and hiding the secondary pie by formatting its borders and fills to None.

True, this chart has improved readability. But it has left out a lot of data simply because (a) it didn’t fit nicely into the chosen chart type, and (b) the data made the chosen chart type too cluttered.

For those of us with data loss aversion, the above chart is even worse than a pie chart.

At the risk of becoming a caricature of myself, I’m going to answer Chandoo’s question “Can I use an alternative to a pie chart?” My answer, if you couldn’t guess, or couldn’t at least see below this paragraph, is a bar chart.

This chart takes up about the same space as either pie chart. Readability has been improved compared to both of Chandoo’s pie charts. The text in the labels is large enough for my eyes, which are twice as old as Chandoo’s, to see clearly. Also, there’s no risk of the color scheme troubling a colorblind viewer.

The bar chart improves on the original pie chart by clearly showing all of the data. No points are hidden by their small size, and the labels are next to the data points, not hanging by a thread.

The bar chart is better than the “improved” pie chart because it keeps all of the data in view, not bundled into a mysterious “Other” category.

Has anybody not seen this unique pie chart? This famous graphic is  purported to compare Sarah Palin’s favorability to that of Mitt Romney and Mike Huckabee. I have seen it dozens of times already, but I think the first place I saw it was in Fox’s Fuzzy Math: 193 Percent Of The Public Support Palin, Huckabee, And Romney.

This was intriguing, so I downloaded the original data (pdf), which after cleaning up and pivoting looks like this:

The data is the result of a telephone poll conducted last month, and shows the favorable and unfavorable ratings of four potential candidates for the 2012 Republican presidential nomination. The Fox report showed only the top three candidates in their pie chart, reproduced below. The intrigue of this chart rests in the sum of 193%, which makes it 1.93 times more effective than your typical pie chart, which adds up to only 100%.

Candidate 4 was not far below, so in fairness, he should be included. This pie chart is even better, since the segments add to 251%.

Obviously the results for these candidates are not exclusive: people are free to favor multiple candidates, so the sums are not expected to add to 100%. Not only is the pie chart a poor data visualization choice in general, even with a data set that shows parts of a whole, in this specific case it’s not even relevant to the data.

So, class, what’s the proper chart type for this kind of data?

Right, a bar chart. This simple shart shows the relative approval ratings of the candidates very clearly. For a typical Fox news audience, this is about as much data as you can afford to show in a half hour of news. As we’ve seen, it’s too much data for a Fox News broadcast team.

For completeness, let’s include poor Newt.

These charts so far only show the results among Republican survey respondents. We get a broader view if we include the responses from Democrats. The data labels were removed in favor of the vertical axis to reduce complexity.

We can even display results from independent respondents without sacrificing any clarity.

Let’s add a weighted average of all responses, using a set of horizontal lines. Using another bar for the averages would be confusing. Note that none of the candidates has an approval greater than 50%.

This chart is too much for a television audience, but it would probably not be too confusing for most readers of national “news” magazines.

The published survey results also included ratings for Oprah Winfrey, who is definitely not a candidate for nomination for any position in the Republican party. I suspect her inclusion is an indication that many people were included in the ratings, but only the top four Republicans were included, and some overworked intern has now been fired because Oprah wasn’t deleted.

The unfavorable and other ratings could also be incorporated into a set of charts, but this is more than enough for the purpose of the newscast.