Every chart has a width and a height, either in pixels on a screen or in physical dimensions on paper or on a projection screen. And those dimensions determine what charts can contain: bigger charts can contain more data, and more and bigger text elements. But it is not only size that matters: the ratio between the width and height of a chart are at least as important for what a chart can show.
Certain chart types tend to grow to a wide layout, while others will quickly evolve to a tall layout.
Bar charts with vertical bars will tend to be wider than high, unless they show only a couple of values. Horizontal bar charts grow in the vertical direction, and have a tendency to be higher than wide.
Source: Maarten Lambrechts, CC BY 4.0
Another example of an inherent aspect ratio is the high layout of slope charts. Slope charts are line charts that show values for only 2 moments in time, so they require only a limited horizontal space. They do need enough vertical space to show the evolution between the 2 moments in time, especially when they contain a lot of time series.
Source: Maarten Lambrechts, CC BY 4.0
And of course, a pie chart is a circle and will occupy a more or less square space in a grid based design (see Grid and arrangement).
Source: Maarten Lambrechts, CC BY 4.0
So if you know the dimensions and the aspect ratio of the space assigned to a visualisation in a publication, this can help you in choosing a chart type that fits the aspect ratio of the available space.
Of course the reverse is more ideal: choose a chart type that fits best the data and the message you want to convey, and reserve a space in your design that has an aspect ratio that fits the chosen visualisation.
One chart that is especially sensitive to differences in aspect ratios are line charts.
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A wide layout, with a high width to height ratio, will flatten trends in time series. A high layout, with a low width to height ratio, will do the reverse and stress or even dramatise trends in the data.
A wide and flat layout suggest slow trends or even flat lines. Source: Maarten Lambrechts, CC BY 4.0
Narrow and high designs exaggerate trends. Source: Maarten Lambrechts, CC BY 4.0
So what is a “good”, “correct” or “honest” aspect ratio for line charts? One often cited rule, called “banking to 45 degrees”, says that the average slope of the lines on a chart should be 45 degrees.
This rule stems from a paper dating back to 1988: But the topic of the paper was how well people were able to compare the slopes of two lines on a chart. So it was not about extracting insights from a single time series, or about seeing the big picture in a line chart with many time series plotted.
On top of that, the researchers did not include average slopes lower than 45 degrees in their study. A follow up study actually showed that people performed better with lower average slopes.