title: How to add a vertical line to a plot from the "mirt" package?
body: <p>I used the <code>mirt</code> package to create an item information plot. </p>
  
  <pre><code>library(mirt)
  model &lt;- mirt(data, 1, itemtype = '2PL')
  plot(model, type = 'infotrace', facet_items = FALSE)
  </code></pre>
  
  <p>I try to add a vertical line to the plot at x = 1. 
  However, the function <code>abline(v = 1)</code> does not work.</p>