title: Labelling every nth tick mark
body: <p>I am plotting an axis with</p>
  
  <pre><code>a &lt;- factor(letters[seq( from = 1, to = 26 )])
  b &lt;- 1:26
  plot(a, b, axes = FALSE)
  axis(1, at = a, labels = a, las = 2)
  </code></pre>
  
  <p>How can I get <strong>all the tick marks</strong> but only <strong>every nth label</strong>, e.g. (every 7th):</p>
  
  <p><a href="https://i.stack.imgur.com/RK2ZI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RK2ZI.png" alt="enter image description here"></a></p>