title: How to draw vertical lines in barplot?
body: <p>I want to draw a vertical line at specific positions of the x-axis, but the x-positions of the lines aren't correct. How can I solve this ?</p>
  
  <pre><code>x &lt;- c(0,0,0,4,5,6)
  barplot(x, names.arg=1:length(x))
  abline(v=1:length(x), col="red")
  abline(v=c(5.5), col="blue")
  </code></pre>