In her book, Story telling with data, Cole Nussbaumer Knaflic states
Sometimes it’s useful to be able to plot data that is in entirely different units against the same x‐axis. This often gives rise to the secondary y‐axis: another vertical axis on the right‐hand side of the graph. When interpreting Fig. 1, it takes some time and reading to understand which data should be read against which axis. Because of this, you should avoid the use of a secondary or right‐hand y‐axis.