title: ggplot2 avoid boxes around legend symbols
body: <p>Consider the example plot below.  I'd like to make the little boxes around each of the symbols in the legend go away.  How an I do this?</p>
  
  <pre><code> ggplot(mtcars, aes(wt, mpg, shape=factor(cyl))) + geom_point() + theme_bw()
  </code></pre>
  
  <p><img src="https://i.imgur.com/f3y7P.png" alt=""></p>