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).
- 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?