title: separate legend geom_smooth in ggplot2
body: <p>I want to make a plot very similar to this one:</p>
  
  <pre><code>library(ggplot2)
  
  ggplot(data=mtcars, aes(x = wt,y =  mpg, colour = factor(cyl)))  + 
    geom_line() + geom_smooth(method = "lm", se=FALSE, lty = 2)
  </code></pre>
  
  <p>However, I specifically want a different entry in the legend for the dashed lines (linear trend) and the solid lines (data). This seems very trivial but for some reason I haven't been able to figure this out.</p>