Reading a graph backwards

A graph is an argument with a picture around it, and the picture is doing more work than most readers notice.

Start with the vertical axis. A chart whose scale begins at zero shows a change in proportion to the quantity. A chart whose scale begins just below the lowest value magnifies the change, sometimes enormously. A rise from 61 to 63 per cent looks trivial on a full axis and dramatic on an axis running from 60 to 64. Neither is a lie. They are answers to different questions — how big relative to the whole, or how big relative to the variation — and the axis decides which question the reader is answering without being told that a question was chosen.

Next, the time range. Any series that moves up and down contains a window supporting almost any conclusion. The honest test is simple: extend the range in both directions and see whether the story survives. Charts that circulate without dates on the axis are frequently charts that would not survive this.

Third, what is being counted. A line labelled 'cases' may count people, tests, or reports, and these move for different reasons. A rise in reports can mean a rise in the thing or a rise in reporting, and only one of those is news.

None of this requires mathematics beyond arithmetic. It requires the habit of looking at the frame before looking at the line, which is the reverse of what the eye does naturally and the reason the frame works so well.