title: R ggplot2: Setting additional specific axis tick marks
body: <p>I have a ggplot:</p>
  
  <pre><code>ggplot()+geom_line(data = data.frame(y = c(1,2,3), x=c(1,2,3)), aes(y=y,x=x))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/McMSh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/McMSh.png" alt="enter image description here"></a>
  I want to keep the default axis breaks and labels (In my program, I do not know the limits of the plot a priori.)</p>
  
  <p>At x= 1.5 I want to add an additional tick mark to the x-axis with label "hi".</p>
  
  <p>I know about <code>scale_x_continuous()</code>, but I do not know how to access the "default breaks computed by the transformation object".</p>