Saturday, 19 May 2012

Good use of pie charts?

FlowingData, one of the people that I follow on twitter, has blogged today about how the visualization below, published by the Wall Street Journal, was a good use of pie charts.


Unfortunately, I have to completely disagree!

I am not against pie charts in a part-to-whole comparison (see QlikTips: Defending Pie Charts) but using them for this type of comparison is not good.

There are two things being encoded by these pie charts:

-  percentage of shares being sold in the IPO - represented in the traditional pie segment fashion.

-  volume of shareholding - represented by the size of the circle.

The first is OK in one single pie chart showing one part-to-whole comparison.  However, the second is a very poor way for us to compare two values - especially when they are not side by side.  Taken together, it is almost impossible to get any real insight from this chart.

As an example, how easy to you find it to compare the 50% pie chart against Tiger Global versus the 6% slice in Mark Zuckerberg's pie?  You can't - it just isn't possible.  The reality is that the 6% is actually much bigger than the 50% in actual share volume.


I would contend that a bar chart is almost always the best medium to represent this type of information.  It is so much easier to interpret and see the values.


Stephen Redmond (@stephencredmond) is CTO of CapricornVentis a QlikView Elite Partner

2 comments:

  1. I absolutely agree with you on this Stephen. This data is much better represented as a bar chart as from a glance you can see the leading figures before even approaching the chart to its numbers. I would say that pie charts are really good for insightful proportionate analysis.

    ReplyDelete
  2. This comment has been removed by a blog administrator.

    ReplyDelete