title: sec_axis function in ggplot2
body: <p>I want to create a secondary axis in <code>ggplot2</code>. I am running the second example in <a href="http://ggplot2.tidyverse.org/reference/sec_axis.html" rel="nofollow noreferrer">http://ggplot2.tidyverse.org/reference/sec_axis.html</a>.</p>
  
  <pre><code>&gt; p &lt;- ggplot(mtcars, aes(cyl, mpg)) +
  +     geom_point()
  &gt; 
  &gt; # Create a simple secondary axis
  &gt; p + scale_y_continuous(sec.axis = sec_axis(~.+10))
  Error in scale_y_continuous(sec.axis = sec_axis(~. + 10)) : 
    unused argument (sec.axis = sec_axis(~. + 10))
  &gt; 
  </code></pre>
  
  <p>Why is this error occurring?</p>