A pie chart is for showing the relative size of different parts of the whole. They are best used for a small number of categories (say up to 10) where you are trying to demonstrate obviously larger values for one or two of the collection of categories. Pie charts are no good if your data doesn’t represent the whole, you have too many categories or the differences between the values is subtle (more on these no-nos below). Just remember that a pie chart is for showing the relative size of parts of a whole collection – the complete pie represents the total.
If you are in any doubt as to whether to use a pie chart – then don’t. Use a column or bar chart instead. Pretty much anything you can show in a pie chart you can show in a column or bar chart (only more clearly). The exception is where one or two of the values is large relative to the others.
Pie charts are much maligned because it is easier to create a bad one than a good one. However pie charts are in common use and therefore most people have a general affinity for them and how to read them. For that reason alone they are worth using. Following are a few tips that I try (and often fail) to follow.
Don’t use too many categories
The most common misuse of a pie chart is when showing too many categories (Figure 1). You can see that the categories in Figure 1 are much easier to comprehend when placed side by side in a column chart (Figure 2). The comparison between Figure 1 and Figure 2 highlights one area where pie charts rule. The long text on the axis labels of Figure 2 are cumbersome and difficult to read because of the orientation. The pie chart by comparison allows you to separate the data visualisation from the legend – giving you more options for layout and styling.
Figure 1: A pie chart with too many categories
Figure 2: A column (or category chart if you prefer) is a better way to display more than ten categories
Groups are too similar to clearly see in a pie chart
Humans are not that great at judging subtle differences in angles but we are pretty good at comparing length. Pie charts are no good if you are trying to illustrate subtle differences. Figures 1 and 2 highlight this well with a lot of similar similar values that you cannot pick up in the pie but can clearly distinguish in the column chart. This is potentially an upside of a pie chart, if you are trying to illustrate that groups are roughly similar by not distracting the reader with subtle differences in bar height then a pie chart might be the way to go.
Pies need to be whole
A pie chart is for showing the relative proportion of components that make up the whole. If the data set is incomplete then don’t use a pie chart.
Hard to compare
Pie charts are for ‘at a glance’ intuitive comparison. Don’t use them when you expect the reader to be making subtle comparisons between pie slices.
Doughnuts Vs Pies
A doughnut /donut chart is simply a pie chart with a hole in the middle. Some would argue that that if pie charts are bad then doughnut charts are pure evil. The argument goes along the lines that the key differentialtor between pie segments is the angle of the segment, so by putting a hole in the middle you make them even more difficult to interpret. I personally don’t agree and for no scientific reason than my own perception. I like the extra white space, to me it makes the chart less cluttered and I don’t have a problem perceiving the pie slice size.
Figure 3: A doughnut chart is just a pie chart with a hole
Sort your pie slices
As with column or bar charts, pie charts are easier to read if the slices are sorted in ascending or descending order. I prefer to arrange the slices in descending order with the largest slice in the top left and descending slices in a clockwise direction (Figure 5).
Figure 4: Unsorted pie slices
Figure 5: Pie slices sorted from largest to smallest and oriented with dominant category on top left – with increasing pie sizes in a clockwise direction.
Making a pie chart in Truii
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