title: ggplot: add manually labelled tick marks on top of automatic tick marks
body: <p>I am trying to highlight the point with the lowest y value by attempting the following:
  1) draw a line from this point down to the x-axis and another to the y-axis; and
  2) add a manual tick mark with this point's x and y value on the x-axis and y-axis, respectively. This manual tick mark must be added in addition to the automatic tick marks on both axes.</p>
  
  <p>Sample data:</p>
  
  <pre><code>df &lt;- data.frame(x=1:100,y=rnorm(100,10,1))
  ggplot(df) +
    geom_point(aes(x=x,y=y))
  </code></pre>
  
  <p>Edit:</p>
  
  <p>Here's an illustration of what I am attempting:</p>
  
  <p><a href="https://i.stack.imgur.com/In3ZK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/In3ZK.png" alt="enter image description here"></a></p>