title: Change secondary line axis color
body: <p>In <code>ggplot2</code>, how can I modify <code>axis.line.y</code> only on the <code>sec.axis</code> (if possible)?</p>
  
  <pre><code>    p &lt;- ggplot(mtcars, aes(cyl, mpg)) +
        geom_point()
  
      # Create a simple secondary axis
      p + scale_y_continuous(sec.axis = sec_axis(~.+10)) +
        theme(axis.line.y = element_line(color = "red"),
              # I can modify text color but not sure about line?
              axis.text.y.right = element_text(color = "red"))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ziwRg.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ziwRg.png" alt="enter image description here"></a></p>
  
  <p>As shown in comments below, full control over right axis elements is now comprised in the development version of <em>ggplot2</em> <code>theme( axis.line.y.right = element_line(color = "red"), axis.ticks.y.right = element_line(color = "red"))</code></p>