Changing the Y-axis scale across graphs – to good effect

In a look at COVID-19 cases across countries, the New York Times changed the Y-axis on the different graphs:

COVID19CurvesAcrossCountries

Typically, readers of graphs should beware when someone changes the scale on the Y-axis; this leads to issues when interpreting the data and can make it look like trends are present when they are not. See two earlier posts – misleading charts of 2015, State of the Union data presented in 2013 – for examples.

But, in this case, adjusting the scale makes some sense. The goal is to show exponential curves, the type of change when a disease spreads throughout a population, and then hopefully a peak and decline on the right side. Some countries have very few cases – such as toward the bottom like in Morocco or Hungary or Mexico – and some have many more – like Italy or South Korea – but the general shape can be similar. Once the rise starts, it is expected to continue until something stops it. And the pattern can look similar across countries.

Also, it is helpful that the creators of this point out at the top that “Scales are adjusted in each country to make the curve more readable.” It is not always reported when Y-axes are altered – and this lack of communication could be intentional – and then readers might not pick up on the issue.

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