On one of my R workshops, someone asked me about creating a ggplot2 with two Y-axes. I do not use such types of plots, because I read somewhere that they have some problems with perception. However, I committed myself to check if it’s possible to create such visualizations using ggplot2. Without a lot of digging, I found this answer from the author of the ggplot2 package on StackOverflow - https://stackoverflow.com/a/3101876. He thinks that those types of plots are bad, fundamentally flawed, and you shouldn’t use them, and ggplot2 does not allow to create them.