souce: Skinnyo.com.

Here is an interesting technique: using two y-axis to display the same data at different resolutions. Yellow (BMI) and blue (Weight) lines should overlap (there is a one-to-one correspondence between BMI and Weight for a given height), but they don’t because they are using a higher resolution for Y2. That’s clever. We can choose to see more detail (BMI) or less detail (kg).

Unless…

Unless they don’t know what they are doing. There are many reasons why you shouldn’t use a secondary y-axis in your charts and this chart proves it. You can use the Y2 axis:

• to replicate Y1 (it helps reading the chart);
• to show one-to-one correspondences (Celsius/Fahrenheit, km/miles)
• Everything else is potentially manipulative.

You must make sure those correspondences are preserved. That’s not the case here: 26.5 is not the right BMI for 74 kg (for this height). If I remove the green line, for the same BMI I get 82.3 kg. So, different correspondences for different resolutions. Hence the title of this post.

I wanted to focus on this, but there are other  issues (terrible smoothing algorithm; is the green line (Goal)  assigned to Y1 or Y2? You can never know).

How do you use the secondary y-axis?