title: In ggplot2, how can I limit the range of geom_hline?
body: <p>Taking a simple plot from ggplot2 manual</p>
  
  <pre><code>p &lt;- ggplot(mtcars, aes(x = wt, y=mpg)) + geom_point()
  p + geom_hline(yintercept=20)
  </code></pre>
  
  <p>I get a horizontal line at value 20, as advertised.</p>
  
  <p><img src="https://i.stack.imgur.com/Lycn3.png" alt="enter image description here"></p>
  
  <p>Is there a way to limit the range of this line on x axis, to let's say2 - 4 range?</p>