How long has it been since I last spotted a rubbish pie chart. Okay, I admit it – it was last month. It is actually quite easy to spot them because actually all pie charts are awful.
So why is this one so bad? Simple. A two-dimensional pie chart has been proven to be hard for humans to accurately compare. We are pretty good at judging simple shapes, like rectangles – but a triangle with a curved side is just a lot harder to judge. When you tilt a pie chart, it makes it even harder because the segments near the top are made smaller by the perspective.
On top of all this, this bizarre example has pie charts being measured not only in degrees-round-a-circle, but they also have different stack heights. While we could have taken a rough guess at the purple slice being about half the size of the green slice in a 2D pie chart, we now have to judge the difference in height – it is twice as height or three times as high? Is the purple segment half the value of the green segment, or just one sixth? Who knows.
It is so easy for a pie chart to be bad that you really don’t need to convert it into a pie-chart-based circular column chart.