In the thick soup of information in everyday life, I’m often swimming in the flotsam of crap graphics and misleading diagrams. A recent edition of the Times of London (22 Apr 09, p. 3) provides not one, but two classic examples in a single graphic sidebar. In discussing the recent £3B profits of the biggest UK supermarket Tesco, the graphic uses a histogram to illustrate the number of Tesco stores worldwide. Normally, a straightforward info graphic, but in this case, one of the values is substantially larger than all of the others. The result is the histogram contains several similarly-sized bars and one outlier that exceeds the available space. To solve this layout problem, some clever designer has slashed the end of the bar as a visual indication that some of it has been snipped out. This technique works fine in situations such as circuit diagrams, where the main task of the reader is to understand connections, and not the area or proximity of components. However, the main function of bars in a histogram is to provide a visual aid for comparison of the relative sizes of the values encoded (number of Tesco stores). The differences among the values can be perceived pre-attentively - they are apprehended all-at-once, without having to make a calculation or even to compare the bars. This “built-in” capability of our visual perception and as such, are extremely powerful for organising and presenting information. By snipping out the missing portion of the top bar, a false perception is created about the true relative sizes of the quantities that have been encoded. A better solution would have been to avoid this entirely, to choose adifferent layout such as a landscape orientation for the callout graphic, or to simply list the numbers as a table.

The second faux pas in this graphical car crash is an illustration of Tesco’s regional sales, using four adjacent circles. These circles are extremely problematic. Several studies have shown that we are very poor at accurately discriminating the relative areas of circles, often falling victim to quite a large error.

These two examples illustrate a common mistake in diagrammatic representation: poor graphical encoding.

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