title: Add a single line in histogram
body: <p>This is an R question.</p>
  
  <p>I want to add a vertical line at the position of mean value in histogram.</p>
  
  <p>My code is:</p>
  
  <pre><code>abline(h=0,v=mg,col="red")#mg is the mean value.
  </code></pre>
  
  <p>But there are two lines come out, except the one I want, also a horizontal line along with X-axis show up.</p>
  
  <p>How can I generate only one line (the vertical line) I want?</p>
  
  <p>Thank you so much for your help!</p>