The most criticized tool in Data Visualization is the pie chart. There are many areas of debate in the world of Data Visualization, but there is little debate among the experts about the pie chart. The number one rule about pie charts is “Don’t Use Pie Charts”. Personally, I’m not offended by them. I understand that it has been the tool of choice and that it has become ingrained into society and business. However, I am in complete support of the expert opinions. Pie charts are deficient in displaying and comparing data. There are a few acceptable uses for them, but in most cases a simple bar chart would be a better tool overall and provide a much better visual comparison.
I have heard people argue that pie charts take up less space or that they are easier to understand, but even these arguments are not valid. There are just too many fundamental problems with pie charts and this is why I advocate that they should not be used. Let’s examine a very simple data set and compare. Here is a table of The Twelve Days of Christmas.
Below is a pie chart of the Twelve Days of Christmas and basically the default view from Excel. To help this visual I’ve followed a common rule of pie charts which is to start at noon and move clockwise from the largest to smallest. The other common practice, as described by Dona Wong in The Wall Street Journal Guide to Information Graphics: The Dos and Don’ts of Presenting Data, Facts, and Figures, is to place the largest slice at noon and the second largest slice to the left of noon and then clockwise with the remaining largest to smaller. I find this practice to be even more confusing, unless the last category is “Other” or “Misc.” and therefore an aggregation of the remaining smaller categories. Also, I added the data to the legend and resized it as large as reasonably possible to make the text readable.
Note the following problems with the pie chart:
• To visually compare the reader must go back and forth from the pie chart to the legend to determine which present matches which color. It would be impossible to list the labels within each slice because the text would be too long. Another popular option is to create lines from the pie chart pointing to each label and place the labels around the pie chart. This creates a very busy chart and clutters the chart with extra lines.
• The use of many different colors is required to create a categorical comparison color scheme. This makes it difficult to see the difference in colors from the shades of blue, red and purple.
• The comparison between the categories is very difficult. The eye cannot easily discern between the size of the “Drummers Drumming” and the “Pipers Piping”. This is because the size of the pie slice is not easily calculated.
• The beginning of one category starts at the end of the previous category. This means that you cannot compare multiple categories from the same baseline, because the baseline shifts from one category to the next.
• Finally, to generate a pie chart it is necessary to calculate the percentage of the categories, after all a pie chart is by nature showing 100% and not 78 total gifts. This may be done manually, but that is not necessary as the software used to create the pie chart will do this automatically (these example charts were built in Microsoft Excel). Now in some cases a percentage might be the correct measure, but in other cases the values may be more appropriate. Below are the calculated fields for what the pie chart is actually showing.
There is nothing wrong mathematically with the pie chart. There are twice as many Geese a Laying then there are French Hens and three times as many Ladies Dancing and French Hens. However, the comparison between these is exactly the point. The pie chart does not make it easy to tell that comparison. It’s hard enough to tell which slice is bigger. It would be impossible to discern twice as much or three times as much.
Here is the same data graphed using a simple bar chart.
This chart solves all of the problems mentioned above.
• Comparisons are made easily from one category to the other because the baseline is now the same for each category. Turtle Doves is clearly twice as many as the Partridge in the Pear Tree. There is no question if there are more Ladies Dancing or Maids a Milking.
• Color is easily managed. There is no color requirement to discern between categories. In fact, this graph could be done in gray scale and printed on a black and white printer or copy machine and it would still be usable.
• The axis labels are now adjacent to the data and the bar. This allows for a very compact chart and is easy to read.
• Finally, unless the pie chart is shrunk to a tiny graphic, for example as a data layer on top of a map, then there is no real space savings. In fact, the bar chart takes up less room on the page and is more readable than the pie chart.
Hopefully this holiday example illustrates the problems associated with using pie charts and the better alternatives. Best wishes for a safe and happy holidays and please keep checking back for more on Data Visualization.