Exploring distance time graphs

1.6.2 Beware of first impressions

Representing ‘sums of money, and time, by parts of space?, as Playfair put it, may indeed seem obvious and readily agreed, but nevertheless graphics showing the rise and fall of profits, expenditure or interest rates over time often need to be approached carefully. As the inventor of the bar chart (or bar graph), Playfair might well have raised a quizzical eyebrow at the example in Figure 32 taken from a national newspaper.

This particular bar chart claims to show how the interest rate on fixed rate loans (often taken out for purchasing or improving a house) has been changing. Look at it for a moment. For what purpose do you think was the graph drawn? How has that purpose been achieved?

The height of each vertical bar represents the interest rate in each period. At first glance, it looks as though the right-hand bar is about twice as high as the left-hand one, giving the impression that the rate had doubled. However, if you look at the vertical scale on the left-hand side of the picture scale, you will see that it starts not at zero but at 5%.

The purpose of the graph seems to be to show that the interest rate in 1994 rose quite substantially between January and May. This effect has been achieved by carefully selecting the range of values shown on the vertical axis. Although this graph is technically correct, it can give a misleading impression if it is not read carefully. If the graph is redrawn as in Figure 33, so that the vertical axis starts at zero, it gives a rather less dramatic impression of the way in which the interest rate changed.

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