The best chart is always task-dependent, but let me assume that you would choose the scatterplot as the best chart and the pie as the worst. They are like water and oil: impossible to mix them!
Let me tell you about a little experiment. I call it the scatterplot pie just for fun, and the idea is to display proportions using a scatterplot.
A traditional pie chart with two data points can be reduced to an angle:
The same message, no fat. And because there are no textures and arcs to deal with we can now superimpose many pies.
These are percentages of the age group 65+ in 1996 (left) and 2050 (right) for 220 countries (data from the US Census Bureau). This comparison clearly shows that the World is getting older.
One of the problems with pie charts is that you can compare proportions but you can’t compare wholes. In the images above we are comparing very different country sizes (Tuvalu and China?). With the scatterplot pie we can add this dimension:
China and India are not helping resolution, but it would happen to any other chart. We can focus on a detail:
Other things we could do:
- Group series (in this example, color-coding by continent would show us the significant differences between Europe and Africa);
- Add axis and circular grid lines to improve readability;
- Set line transparency to 50%;
- Remove the vertical line or make it look like a grid line;
- Label the more significant data points.
I actually like this idea and I’ll test it a bit further. I’ll try to decide if it is a good alternative to stacked bar charts. It should also work with three or more slices, but just because it works it doesn’t mean we should use it (like most chart options in Excel…).
So, what do you think? Would you use it?
This is not entirely mine: I was inspired by this comment. And I’m sure someone must have though about this first. If it rings a bell please let me know…)