title: Boxplot one x axis tick mark label for two boxes
body: <p>I'm trying to create a boxplot using ggplot2 in R, below is my code and the plot it produces. I want to change it so instead of having the x axis labelled as 0.5mg, 0.5mg, 1mg, 1mg, 2mg and 2mg, I want just 0.5mg, 1mg, and 2mg between each of the sets of two box plots. Is there any way to do this?</p>
  
  <p><a href="https://i.stack.imgur.com/kKCyQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kKCyQ.png" alt="boxplot"></a></p>
  
  <pre><code>ggplot(ToothGrowth, aes(x=interaction(supp, dose), y=len, fill=supp)) + 
  geom_boxplot() +
  scale_x_discrete(labels = c("0.5mg", "0.5mg", "1mg", "1mg", "2mg", "2mg"), name = "Dosage") +
  scale_y_continuous(name = "Tooth Length") + 
  scale_fill_discrete(name = "Supplement",
                      labels = c("Orange Juice", "Ascorbic Acid"))
  </code></pre>