title: How to add a 2nd y-axis for a density plot superimposed on a histogram in R?
body: <p>I have a histogram shown below, and I have added 2 density plots on top of that. It corresponds to the 2 classes that make up the data. </p>
  
  <p><img src="https://i.stack.imgur.com/enqfp.png" alt="histogram">
  I want to add a 2nd y-axis on the right, but having same height as the 1st y-axis, so that the height of the density plots do not look so small. <strong>The relative heights of the 2 density plots must to directly comparable, such that their combined area is 1</strong></p>