title: Boxplot add line to a single box
body: <p>I am plotting the local clustering (<code>transitivity</code> with <code>type = c('local')</code> argument) of some graphs in a <code>boxplot</code>. </p>
  
  <p><img src="https://i.stack.imgur.com/79g0L.png" alt="plot"></p>
  
  <p>I calculated the <code>global</code> transitivity of these graphs and I want to represent it as an additional line of the plot as wide as each box.</p>
  
  <pre><code> df &lt;- data.frame(values = c(full_trans, clus_trans), vars = rep(c("Full","Clustering"), times=c(20,20)))
   par(mfrow=c(1,1))
   boxplot(values~vars, data=df, ylim=c(0:1), yaxt='n', boxwex=0.5)
  </code></pre>
  
  <p>I tried <code>abline</code>, but it draws a line through all the plot which doesn't look how I wanted.</p>