title: Duplicate and customize secondary y axis
body: <p>This should be simple but I am getting some errors. I want to duplicate and then customize the labels of the secondary y axis.</p>
  
  <p>First, this simple code should yield the following figure:</p>
  
  <pre><code>  ggplot(data = mpg, aes(x = displ, y = hwy)) + 
        geom_point() +
        scale_x_continuous(sec.axis = dup_axis()) +
        scale_y_continuous(sec.axis = dup_axis())
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/lUMbt.png" rel="noreferrer"><img src="https://i.stack.imgur.com/lUMbt.png" alt="enter image description here"></a></p>
  
  <p>But I don't know why I get the following error (it works without the <code>scale_</code> arguments and I have ggplot2 version 2.2.1):</p>
  
  <p><code>Error in .Call(rhs, f) : first argument must be a string (of length 1) or native symbol reference</code></p>
  
  <p>Second, once the y axis is duplicated in right side, I would like to change the tick labels (20, 30, 40) for, let's say, ("a", "b", "c").</p>
  
  <p>How can I fix that error and customize the tick labels of the secondary y axis?</p>