Dual axes, where there are two value scales in a single chart, are almost never a good idea. As a reader, you should always question the source when you see a chart that uses such scales. Zan Armstrong explains with a recent example.

One of the best descriptions I’ve heard for data viz is that: when the data is different, the viz should look different and when the data is similar, the viz should look similar.

If you allow yourself to have two y-axis for the same metric, with both a different scale on each axis and a different base value, then you can make a lot of charts with the exact same data that look very different.

If there’s a direct transformation between the scales, say between metric and Imperial units, then okay, that’s fine. In almost all other cases, people use dual axes to overemphasize a relationship between two variables, and you should wonder why the maker did that.

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Nathan Yau
http://flowingdata.com

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