title: Annotate ggplot with an extra tick and label
body: <p>Can you help me annotate a ggplot2 scatterplot? </p>
  
  <p>To a typical scatter plot (black):</p>
  
  <pre><code>df &lt;- data.frame(x=seq(1:100), y=sort(rexp(100, 2), decreasing = T))
  ggplot(df, aes(x=x, y=y)) + geom_point()
  </code></pre>
  
  <p>I want to add annotations in the form of an extra tick and a custom label (red):</p>
  
  <p>Example image:</p>
  
  <p><img src="https://i.stack.imgur.com/qlZXD.png" alt="enter image description here"></p>