title: qqline in ggplot2 with facets
body: <p><a href="https://stackoverflow.com/questions/4357031/qqnorm-and-qqline-in-ggplot2">This</a> question showed how to make a qqplot with a qqline in ggplot2, but the answer only seems to work when plotting the entire dataset in a single graph.</p>
  
  <p>I want a way to quickly compare these plots for subsets of my data. That is, I want to make qqplots with qqlines on a graph with facets. So in the following example, there would be lines for all 9 plots, each with their own intercept and slope.</p>
  
  <pre><code>df1 = data.frame(x = rnorm(1000, 10),
                   y = sample(LETTERS[1:3], 100, replace = TRUE),
                   z = sample(letters[1:3], 100, replace = TRUE))
  
  ggplot(df1, aes(sample = x)) +
    stat_qq() +
    facet_grid(y ~ z)
  </code></pre>
  
  <p><img src="https://i.imgur.com/q11bnTf.jpg" alt="facet data"></p>