title: Increase the length of plot tick marks
body: <p>How can I increase the length of plot tick marks? Here is a small example:</p>
  
  <pre><code>r &lt;- as.POSIXct(round(range(time), "mins"))
  plot(time, x, t="l", xaxt = "n")
  axis.POSIXct(1, at = seq(r[1], r[2], by = "min"), format = "%H:%M:%S")
  </code></pre>
  
  <p>which gives</p>
  
  <p><img src="https://i.stack.imgur.com/ewYaJ.png" alt="enter image description here"></p>
  
  <p>As you can see, all the ticks are the same size. Is there a way to automatically increase the length of those ticks that are signed? </p>