title: create boxplots with transparent colour ggplot2
body: <p>I create a graphic with several groups and plotting a geom_boxplot() over a seet of lines.
  However, it would be nice to colour the boxes transparently so that the lines can be seen.</p>
  
  <p>Here's some sample data:</p>
  
  <pre><code>x11()
  
  name &lt;- c("a", "a", "a", "a", "a", "a","a", "a", "a", "b", "b", "b","b", "b", "b","b", "b", "b")
  class &lt;- c("c1", "c1", "c1", "c2", "c2", "c2", "c3", "c3", "c3","c1", "c1", "c1", "c2", "c2", "c2", "c3", "c3", "c3")
  year &lt;- c("2010", "2009", "2008", "2010", "2009", "2008", "2010", "2009", "2008", "2010", "2009", "2008", "2010", "2009", "2008", "2010", "2009", "2008")
  value &lt;- c(100, 33, 80, 90, 80, 100, 100, 90, 80, 90, 80, 100, 100, 90, 80, 99, 80, 100)
  
  df &lt;- data.frame(name, class, year, value)
  df
  </code></pre>
  
  <p>I draw the graphic with:</p>
  
  <pre><code>p1 &lt;- ggplot(df, aes(year, value))
  p1 &lt;- p1 + geom_line(aes(group=name, size=name),colour="#ff2300",alpha=0.5) +     facet_wrap(~ class, scales = "free_y") 
  p1 &lt;- p1 + geom_boxplot(aes(group=name))
  print(p1)
  </code></pre>
  
  <p>And on my system the line width in the legend is not correctly displayed. Am I doing something wrong? Thanks in advance!</p>