Aspect Ratio and Banking to 45 Degrees

The same data can look very different in a line chart depending on its aspect ratio. But what is the perfect shape for a chart? A square? A rectangle? Which rectangle? It depends on the data.

The ratio between the width and the height of a rectangle is called its aspect ratio. It is typically expressed as a fraction with two numbers, the width divided by the height. An aspect ratio of 1:1 describes a square, while 4:3 (or 1.33:1) is a landscape rectangle, and 16:9 is a much wider landscape rectangle. While the width is usually larger than the height in film and photography, there is no reason that this be the case in other applications like charts.

When applied to visualization, the aspect ratio describes the area that is occupied by the data in the chart, even if the overall chart area might be larger. A change in aspect ratio means a change in the angle of the lines, etc.

The image below shows the same graph (which you may remember from the posting on baselines) in three different aspect ratios. Note how the same parts of the graph have very different slopes in the different versions.

In contrast to a photograph, where a wrong aspect ratio is usually easy to detect because the image appears stretched or squished, things aren’t nearly as obvious in a chart. What is the right aspect ratio?

In a paper from 1988, Bill [...]