Source: U.S. Census Bureau (original Excel file). The abortion ratio is defined by the number of abortions per 1,000 abortions and live births.
(Click to enlarge)
1. We know that information visualization is all about pattern detection. But often our design choices hide relevant patterns behind the obvious one(s). Take this panel, for instance. Everyone can see the downward pattern, but what about the U-shaped pattern across age groups? You can see it, right? Well, follow the usual path and you’ll miss it.
2. A ratio (or a growth rate) is something that should always be put in the context of actual volumes or proportions. There is no “best answer” to link both dimensions but the panel displays a reasonable solution. As you can see, the abortion ratio among women less than 15 years old is very high but its proportion in the total number ob abortions is almost residual. On the other hand, the ratio in the 15-19 age group may be lower, but it is much higher than the average ratio and accounts for around 17 of the total abortions. Whenever possible, you should keep these two measures close together.
3. There are seven age groups in this data set. Put them all together in a single line chart and you’ll miss the pattern across groups, as discussed above, but you’ll also have a hard time disentangling the whole thing. Before creating the chart always ask: what is my specific question? This will help you to prioritize and create a focus-context display. If a series answers your question (need-to-have) add it to the chart and color-code it. If a series is interesting but doesn’t directly answer your question (nice-to-have), you may add it to the chart to provide context, but gray it out and delete if from the legend, if you have one.
Please let me know what you think and suggest ways to improve the panel.
[Update: Jon discusses the process of pattern discovery in RE: Abortion Ratios 1980-2003. Andreas adds several good suggestions and shows how to display the date in a more consistent small multiples chart.]