A Great Example Of A Terrible Pie Chart
Monday, 3rd October 2011
If you are going to use a pie chart (please don't) then you need to know the rules of the pie chart. I actually don't think anyone ever needs to use a pie chart, they are very nearly always the wrong chart for the data you want to display and despite the fact that you now think that your chart is in the small percentage of charts that would work as a pie chart you are actually wrong and should think about perhaps using a column chart, or a pie chart or a line graph. Not as sexy as a pie chart, but much better for displaying data.
So with this in mind, what are the rules of the pie chart, and just how many of them does this chart break?
I would like to point out that I didn't create this chart, it was displayed during a presentation and I took a hasty photo with my smart phone in order to share it with you. This is also the reason why I appear in the reflection on the display screen!
Type Of Data
The one argument in favour of pie charts is that it succinctly shows that the data is part of a whole. For example, each slice represents a percentage of an entirety, for example "out of the 100 people we asked, 50% of them said yes, 30% said no and 20% said maybe". Because these are all plotted on a circle we can see all four values (50, 30, 20 and 100).
You don't actually need to use a pie chart to demonstrate this, but this is one of the most reasonable arguments for using a pie chart.
It is not advisable to use a pie chart to plot data that doesn't make up a whole. It is also a bad idea to use a pie chart to show data over time. A line graph or column chart is much better suited to this.
Error number one, this pie chart shows data over time.
Most charts come with measurements for free. On a line graph you get two - one up the side and one along the bottom usually. On these lines you normally have a label that tells you what is being measured and values to represent the units of measurement.
On a pie chart there is no natural measurement or zone for this information. Sometimes people handily pop the values on each segment, which works if the segments are big enough to hold the number. Quite often these numbers are not accompanied by what is being measured and more often than not the labels that tell you what each segment represents is a list of coloured blocks that match the colours used on the pie chart. Bearing in mind how common colour-blindness is, using colour alone to show information is a very bad idea.
Error two, no labels to show the value of each segment.
Error three, using colour alone to label segments.
Can I Take Your Order
Getting the order right is important for all charts. For column charts and line graphs you very often put things in chronological order, or whatever order suits the data. For example, if you had results from a survey, you might opt to put them in order of popularity from highest to lowest. If you were showing stock prices over time, you would certainly start at the earliest date move sequentially forward in time.
With pie charts, it really helps if you put your segments in order, starting with the highest. It also helps if you put your labels in the same order as the segments, starting at the top and working clockwise. Not only does this help to match the segments to the labels, it also aids comparison of the segments. If two similarly sized segments appear close together, it is easier to determine which is the smallest by the order. This actually highlights one of the problems with pie charts - it is harder to compare similar values on a pie chart than on column, line or bar charts.
The ordering on this chart is chronological, which we have already agreed is bad for pie charts, but it does make it harder to order and compare the results (you have to map the segments to the the label, then sort them mentally - including the similarly sized segments that are hard to determine the order for.
Error four, not ordering segments by value, highest to lowest.
Error five, the segments are actually arranged anti-clockwise, which will make it even harder for the colour-blind people to interpret.
What Time Is It?
Another piece of important advice for pie-charts is that you should start the first segment at 12 o'clock. It is easier to interpret the values of segments that start at 0, 90, 180 and 270 degrees. It is unlikely that subsequent values will start at these points, but you can help everyone out by starting the first value at 0 degrees!
If you imagine a pie chart with three values, 50%, 25% and 25%, you wouldn't need any value labels to know those numbers - it would be really obvious. Sadly, real life data often fails to divide itself nicely right-angled segments.
Error five, not a single value is placed on 0, 90, 180 or 270 degrees.
Too Much Information
Pie charts stop working entirely when you add too many segments. The safe bet is to stick the first four segments on and then gather up everything else into an "Other" segment. Attempting to display every segment of information often ends up in a car-crash of pie chart labels.
Error six, too many segments.
That is surely plenty to be getting on with. The main error is actually using a pie chart in the first place, but it really doesn't help when you ignore all of the sensible conventions for using a chart. If you broke this many rules with any chart it would look terrible, but it is so easy to mess up a pie chart because it is fundamentally flawed in the first place.
Let's just imagine what things might have looked like if we had just used a more appropriate chart in the first place - for fairness I have styled this to also look like it was made in the eighties!