Donuts: Still inedible
Jul 31, 2005
Concerning my post on the Economist's use of donut charts, Phillip on Blogcritics raised two issues worth further study.
First, are pie and donut charts "natural" for representing percentages? In my opinion, if only one set of percentages is involved, then either a table of numbers or a "decile chart" works much better.
The "decile chart" would look better if I have used 10 human figures, one for each 10% of the population.
This "decile chart" addresses the visual estimation issue Phillip brought up. Because each dot / human figure represents 10%, even if the percentages are not annotated, the reader can gauge them visually. Not so for pie slices: no one will be able to tell a 15% slice from a 10% slice.
Besides, the point of the original graphic was to compare percentages. The message is that the white population is expected to decline while the Hispanic, Black and Asian populations would increase. When two pies (or donuts) are used, the reader is tasked with differentiating a 67% slice from a 58% slice situated an inch apart. That, I submit, is a tall order. By contrast, the growth rate is explicitly coded into the gradient of the lines in my junkart: the steeper the line, the higher the growth (or decay).
Second, what if growth rates are chaotic and lines criss-cross each other? This presents no problem at all:
The first line chart shows two segments increasing at the same rate and one segment declining fast. The second chart shows two segments dropping at different rates and one segment skyrocketing. Because the growth rate is explicitly plotted, the reader has no problem picking it up.
The astute reader will note this chart looks like the marvellous Bumps chart.
Phillip also noticed another feature of the Economist chart that escaped me: the two donuts were sized proportional to the total populations in 2004 and 2030 respectively. Ouch! Now, the area of a slice depends on both angle and radius, making it nigh impossible to compare them.
Thanks Phillip for pushing my thoughts on this.
Your site is great, I really like your posts on bumps charts. Junkcharts is the only site besides Tufte where I have found anything about bumps.
A few questions:
What should you call a bumps like chart that is used for non crew related data?
Are you aware of any other examples, sites, references to bumps?
Why aren't bumps used for more before - after comparisons?
Any informatiopn on bumps will be appreciated.
Posted by: Kelly O'Day | Jun 17, 2006 at 01:10 PM
Thanks for the comments. If you try "bumps" in the search box, you'll find quite a few occasions where I suggested using Bumps charts.
I've been calling them Bumps charts, not sure if there is an official name for it. Essentially it is perfect for tracing changes in ranks over time.
I've extended its usage to tracing changes in value over time but this is tricky especially if the values are bunched together.
Posted by: Kaiser | Jun 19, 2006 at 10:28 PM
I've made a page on my site for bumps charts. Once you start lookng, there are a lot of charts out there that coud be improved by using a bumps format.
I'll be interested any thoughts you may have on these examples.
Posted by: Kelly O'Day | Jun 20, 2006 at 04:07 PM
You note "Not so for pie slices: no one will be able to tell a 15% slice from a 10% slice."
I'm guessing one or more of the following statements is false:
Your mother baked a lot of pies.
You had several siblings, especially brothers.
If both of the above were true, you'd be easily able to tell the difference between a 10.0% slice of pie and a 10.1% slice of pie. The bigger slice is, obviously, the one your brother is unfairly getting.
[In terms of pie charts, though, you are correct.]
Posted by: zbicyclist | Jul 14, 2006 at 10:21 PM