title: Controlling the 'alpha' level in a ggplot2 legend
body: <p>In ggplot2, how can I make the legend have a semi-transparent background.</p>
  
  <p>The following code, gives a fully transparent background (and positioning control)</p>
  
  <pre><code>plot &lt;- plot + theme(legend.position=c(1,1),legend.justification=c(1,1),
                         legend.direction="vertical",
                         legend.box="horizontal",
                         legend.box.just = c("top"), 
                         legend.background = element_rect(fill="transparent"))
  </code></pre>
  
  <p>But how can one control the level of alpha, I don't believe that <code>element_rect</code> has that ability.</p>