title: plot line behind barplot
body: <p>I would like to create a barplot where the bars are plotted on top of the horizontal line.</p>
  
  <p>The following code accomplishes this:</p>
  
  <pre><code>y &lt;- c(1,2,3,5)
  barplot(y)
  abline(h=mean(y))
  barplot(y, add=T)
  </code></pre>
  
  <p>However, I'm concerned that the <code>add=T</code> parameter in <code>barplot()</code>, if used repeatedly, can introduce printing artifacts.  I'm curious if there's an alternative to the above code (though the above code may be the fastest method).</p>