title: Plot dashed regression line with geom_smooth in ggplot2
body: <p>I have a simple plot in ggplot2 and want to add a dashed regression line.  So far I have:</p>
  
  <pre><code>library(ggplot2)
  ggplot(mtcars, aes(x = hp, y = mpg)) +
    geom_point() +
    geom_smooth(method = "lm", se = FALSE) +
    theme_bw()
  </code></pre>
  
  <p>Which returns what I want, but with a solid line:</p>
  
  <p><a href="https://i.stack.imgur.com/uPV6x.png" rel="noreferrer"><img src="https://i.stack.imgur.com/uPV6x.png" alt="scatterplot with regression line"></a></p>
  
  <p>I want to make the line dashed.  I think I should use <code>scale_linetype_manual()</code> but my attempts have been hacky.</p>
  
  <p>A simple question, but I couldn't find a duplicate.</p>