Labelling every nth tick mark
<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>