16 Mar 11 A quick word on pie-charts
For those of you who follow Stephen Few, many of you will be familiar with his opinions on pie charts. I whole heartedly agree with him, and I’d like to thank Stephen for illuminating many of the foundation principles of data visualisation and dashboard design at his course I recently attended in Melbourne. Essentially what Stephen says about pie charts is that we have great difficulty comparing areas, and we are very good at comparing lengths, so when making a part to whole comparison, use a bar chart - or if you are a little more sophisticated a Pareto chart. He demonstrates this very well here;
Upon leaving that course, I thought about this quite a bit in light of all the other ways he had taught us to simplify any data visualisation and strip it down to its purest form. The minimal information that it was conveying. The more I thought about this the more I thought that there was one piece of information in the pie chart that was not easily presented in the bar chart - the fact that you are looking at a part-to-whole relationship. When the maximum value on the X-axis in this example is 50%, it isn’t immediately apparent that adding up A to F would result in 100% - it is however in the pie chart. So I thought long and hard about how to resolve this piece of missing information. Behold, the Pareto chart which Stephen showed me, which essentially does this.
Again, there is a trade off here. By including the line to indicate the contribution to the whole we can quite easily read a cumulative total and see individual contributions. I guess you do lose a little bit of clarity for the uninitiated, as to what this is exactly - given that there is a line and bars such that it isn’t immediately apparent that they are related pieces of data. You also have a scale problem which makes it slightly harder to compare lengths when you necessarily need to make the axis go to 100%. Despite these drawbacks, I’m a big fan of the Pareto chart.
Finally, I thought that, given the fact that we now read everything on a screen, be it computer, tablet or e-book, would it be possible to get all the benefits of the bar graph, and communicate the part-to-whole relationship using animation. The stacked bar graph clearly shows a part to whole relationship, but makes it incredible difficult to compare length. If you could animate it to disassemble into a normal bar graph, you would have the best of both worlds. Eg Animate this;
to this
This is really only applicable in a Powerpoint presentation as it wouldn’t make any sense in a dashboard or report, and you do however lose what Pareto offers in assessing a cumulative total. Despite this, I think it may be an effective intermediary between Pareto and the bar chart that retains the one useful piece of information in a pie-chart.
