title: lower and upper quartiles in boxplot in R
body: <p>I have</p>
  <pre><code>X = c(20 ,18, 34, 45, 30, 51, 63, 52, 29, 36, 27, 24)
  </code></pre>
  <p>With <code>boxplot</code>, i'm trying to plot the <code>quantile(X,0.25)</code> and <code>quantile(X,0.75)</code>
  but this is not realy the same lower and upper quartiles in boxplot in R</p>
  <pre><code>boxplot(X)
  abline(h=quantile(X,0.25), col=&quot;red&quot;, lty=2)
  abline(h=quantile(X,0.75), col=&quot;red&quot;, lty=2)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/ONL8w.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ONL8w.png" alt="enter image description here" /></a>
  Do you know why?</p>