A single chart with multiple Y-axes is usually used to show a relationship between trends or to save some space on slides. This way of working might seem like a perfect solution, but we believe you’re better off without it.
In the example above, a color code and label are used to indicate which line belongs to which axis. This is a handy trick, but most often forgotten. Also imagine such graph in a paper printed in black and white. You might be looking at the wrong side.
You also need a trained neck that you can twist and turn as such you can read the vertical text on the axes. Do you feel the pain already?
It is a great idea to indicate the relationship between two things, but the line chart is not the way to go for that. Putting them in the same graph introduces confusion points, like the crossing part in the example above.
When you look at the scale of both y-axes, you see that they are not the same. This is seen in most cases and affects the interpretation which might end up in wrong findings. Putting two variables in the same graph visually implies more relationship between them than actually exists and the crossing point is meaningless.
Since you don’t want your viewers to be misled, it is better to use an alternative.
Unless the scale of the two y-axes is the same, we believe it is better to put the two lines in two separate graphs next to each other. As such, it is still easy to spot if there is any possible relation between the two of them without having the complicated intersection.