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Visualisation: visual representations of data and information
Visualisation: visual representations of data and information

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3.2 Cheating with bar charts

Bar charts are subject to various sorts of ‘creative’ use. For example, the bar chart in Figure 7 shows huge differences in the four charted quantities, does it not?

Or maybe not – see also Figure 8.

A bar chart
Figure 7 The values 235, 255, 270, 240 shown on a bar chart
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This vertical bar chart has 4 columns of varying height. The horizontal axis has no label and just shows each column labelled with a, b, c and d from left to right. The vertical axis is not labelled and has no units, but the scale goes from 200 to 280 in divisions of 10. The heights of the columns cannot be read accurately but are at approximately the following heights on the vertical axis:

  • a 235
  • b 255
  • c 270
  • d 240

The overall impression is of quite large variation in bar heights.

Figure 7 The values 235, 255, 270, 240 shown on a bar chart
An alternative bar chart
Figure 8 The values 235, 255, 270, 240 shown on a bar chart, but this time with the vertical axis starting at zero
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This vertical bar chart has 4 columns of varying height. The horizontal axis has no label and just shows each column labelled with a, b, c and d from left to right. The vertical axis is not labelled and has no units, but the scale goes from 0 to 300 in divisions of 50. The heights of the columns cannot be read accurately but are at approximately the following heights on the vertical axis:

  • a 235
  • b 255
  • c 270
  • d 240

The overall impression is of quite small variation in bar heights.

Figure 8 The values 235, 255, 270, 240 shown on a bar chart, but this time with the vertical axis starting at zero

Many spreadsheet packages that are used to create charts also allow the user to employ shapes other than simple bars when constructing a bar chart. This may not be a good thing.

For example, chart widgets like the ones shown in Figure 9 are available from Google Charts. As well as being potentially misleading because it’s not immediately clear where zero lies (the train chart ranges from 200 to 270 whereas the piles of money chart ranges from 0 to 270), the imagery can also be a distraction. Where different 2D shapes are used for the bars, the area of the shape may change out of proportion with the height or length of the ‘bars’, which would mislead the reader at a perceptual level. Where 3D imagery is used, the reader can be confused (even unconsciously) about whether the height or the volume of the chart is what is significant.

Data represented by the length of a goods train and by piles of bank notes
Figure 9 (a) The values 235, 255, 270 and 240 represented by shapes. Here the different lengths of the two shapes used can mislead (as can the fact the ‘bars’ do not start at zero). (b) The same values represented by piles of money. Here the fact that there is a 3D representation is misleading: is it height or volume that represents the four values?
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This figure shows 2 charts designed with images of real items representing the columns.

Chart (a) is similar to a horizontal bar chart. Each bar consists of a train locomotive and some carriages. Each bar has a label showing its length (although there are no scales). From the top the lengths are:

  • A 235
  • B 255
  • C 270
  • D 240

Chart (b) simply shows some piles of money (notes). Although differences in height of the money piles can be noted it is only possible to make very rough judgements about their relative heights. The piles are labelled from left to right:

  • A 235
  • B 255
  • C 270
  • D 240
Figure 9 (a) The values 235, 255, 270 and 240 represented by shapes. Here the different lengths of the two shapes used can mislead (as ...