## Circular but insufficient

##### Nov 13, 2014

One of my students analyzed the following Economist chart for her homework.

I was looking for it online, and found an interactive version that is a bit different (link). Here are three screen shots from the online version for years 2009, 2013 and 2018. The first and last snapshots correspond to the years depicted in the print version.

The online version is the self-sufficiency test for the print version. In testing self-sufficiency, we want to see if the visual elements (i.e. the circular sectors on the print version) pull their own weights. The quick answer is no. The reader can't tell how much sales are represented in each sector, nor can they reliably estimate the relative scales of print versus ebook (pink/red vs yellow/orange) or year-to-year growth rates.

As usual, when we see the entire data set printed on the chart itself, it is giveaway that the visual elements are mere ornaments.

The online version does not have labels unless you hover over the hemispheres. But again it is a challenge to learn anything from the picture.

In the Trifecta checkup, this is a Type V chart.

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This particular dataset is made for the bumps-style chart:

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A group of my students used 5 pie charts to show changes over time in their stat consulting report. Luckily they had some other sensible graphs or it would have been 0 out of 5 for the graphing mark. Worst thing was that it would have been easier to create a line chart than manipulate the 5 pie charts.

Just commenting on the print version.

I think this is a bad example of why pies are bad because here it actually works quite well.
In some ways better than the bumps-style chart.

It is not pie chart by the way but a coxcomb chart.

The coxcombs show relations between sales. The use of area is not too bad as there is some very small and some very small numbers.

The bumps chart show another set of data, the relation of the differences between the sales. The lack of a clear zero baseline is strange and makes this point very clear.

Pies (or coxcombs) and area scale are often bad but no rule without an exception.

@Jörgen - I couldn't disagree more with your assessment.

Whatever you want to call this set up, what we have are 4 bubbles, but all cut into quarters, which takes a poor way of comparing data and makes it harder...

All you can really tell in the print chart is that some numbers are big and some are small. Getting a real idea of how much bigger or how much smaller is very difficult.

The bumps chart makes it very clear how much a value has changed between the time periods in question, and the slopes make it very clear how each category compares to the others in terms of the severity of change.

It also does this at a glance, as opposed to the bubble charts where you need to look at each category, compare it's first and last values, and then move on to the next, trying to keep in mind the first values you were looking at...

I don't see any possible way to defend that as a good way to display the data to the reader.

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