With all the visualization options out there, it can be hard to figure out what graph or chart suits your data best. This is a guide to make your decision easier for one particular type of data: proportions.
Maybe you want to show poll results or the types of crime over time, or maybe you're interested in a single percentage. Here's how you can show it.
We all know about the pie chart. The circle represents the whole, and the size of wedge represents a percentage of that whole. Together, those represented values, add up to 100 percent. Use this only if you're comparing a few values (like three or less) or if you're like me, use it for a ton of categories to annoy the BI people every now and then.
See the pie in action: What Do You Use to Analyze and/or Visualize Data?
Oh yes, it's pie's lesser-used cousin, the donut. It's the same idea as the pie, but with a hole cut out in the middle. The same arguments of angles and human perception still apply (probably more so). I personally don't remember using the donut ever, and can't think of why I ever would. But I suppose it has its uses somewhere.
See the donut in action: What Britain Has Eaten the Past Three Decades
Use the stacked area chart if you want to show changes over time for several variables. You can use it for percentages, where the vertical always adds up to 100 percent, or you can use raw counts if you're more interested in the peaks and valleys.
See the stacked area in action: (Baby) NameVoyager
If you have only a few distinct points in time, you can use the stacked bar chart in the same way you use the stacked area (just set the bars vertical). You can also use them as you would a pie chart, and it's usually a better option because it's sans angle perception problem.
See the stacked bar in action: New York Times Poll Watch
Now we're getting into more advanced stuff. The treemap uses the areas of rectangles to show relative proportions. It works especially well if your data has a hierarchical structure with parent nodes, children, etc.
See the treemap in action: The Google Newsmap
Again we're using area to visualize magnitude, except instead of rectangles or wedges, a Voronoi diagram uses polygons. The argument for the Voronoi is a more robust algorithm that is able to sidestep some of the problems when restricted to rectangles.
See the Voronoi in action: American Consumers Spend More Money On Cheese than On Computers
The Nightingale rose graph (or the polar area diagram if you like), coined after its creator, Florence Nightingale, is like a combination of the stacked bar and pie chart. The length of radius is used to indicate one thing, usually a count, and polar area represents a portion of the whole.
See the Nightingale in action: The original mortality chart from Florence Nightingale
Designers like this one a lot when they want to focus on a single data point. Statisticians not so much. It takes up a lot of space, but sometimes puts things in better perspective. Basically instead of showing each data point, you're showing every individual count within a data point.
See the everything in action: The Cost of Higher Education
There are plenty of other methods to visualize proportions, but all others that come to mind are variants of the above. Of course, if none of these do it for you, you can always turn to your standard table. Forget the pictures or visualization, and just let the numbers do the talking.
Did I miss anything? Do you know of any other good examples that visualize proportions? Let us know in the comments below.