This chart, from Internet Retailer (March 2012, p. 26), is okay, at least they didn't use pie charts. But it could have been much more effective.
To make it better, we have to break all the rules:
- Use lines instead of columns. The following reproduces the right side of the chart above which deals with shopping behavior by income groups. One of the issues with this chart is that the gray partition between the gender section and the income section is too incognito.
- Use rounded percentages instead of two decimal places. If we need to see two decimal places in order to tell two categories apart, then the difference is surely insignificant.
- Round up group boundaries. Notice that I committed the sin of imprecision by not specifying whether $60,000 belongs to $30-60K or $60-100K. Is that level of precision necessary? Is it worth the ugliness of printing a number like $59,999? If there is information in whether $60,000 belongs to $30-60K or $60-100K, what does that say about your analysis?
- Dare to throw away information. Look at this final version of the above charts:
I have collapsed the five income groups into two. That's because the bottom three income groups are more or less the same in terms of online/offline shopping behavior, and the top two groups are more or less the same. Bear in mind that any analysis of this type has a margin of error so differences of a few percentage points are not worth representing. It is possible that even the 10 percent or so differences between the two remaining income groups are not meaningful--we won't know unless we know the margin of error of their methodology.
(Note that you have to consult the Census to get the relative proportions of each income group in order to average the data shown in the original chart.)
In any case, the key message of the article becomes a lot clearer than in the original chart.
- Use black and white instead of color. Having simplified the presentation--but retaining all the important information--we no longer need colors.
Start breaking some rules today, and you'll make better charts.