title: Missing X axis values on R plot
body: <p>Can someone tell me why this simple reproducible example yields missing values on the X axis?</p>
  
  <p><code>at</code> is supposed to put tick-marks from 1 to 20 right?</p>
  
  <pre><code>test &lt;- iris[,3]
  test &lt;- test[1:20]
  plot(test, type = "o", pch = 19)
  axis(1, at=c(1:20), labels= c(1:20))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/HFQOr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HFQOr.png" alt="enter image description here"></a></p>