title: multiple outliers in ggplot -> alpha to outlier.color
body: <p>I want to set my outlier points in a boxplot to be semitransparent
  In <a href="https://stackoverflow.com/questions/3010403/jitter-if-multiple-outliers-in-ggplot2-boxplot">here!</a> they used "jitter" (similar idea, different approach)</p>
  
  <p>my code</p>
  
  <pre><code>ggplot() + 
  geom_boxplot(aes(x = Sistema, y=values, linetype = Sistema), 
               data=estacado, outlier.size=1, outlier.shape=2) + 
  coord_flip() + 
  labs(x="Sistema", y=expression(paste("RMSD ",(ring(A))))) +
  opts(legend.position="none")
  </code></pre>
  
  <p>my data</p>
  
  <pre><code>&gt;head(estacado)
      values   ind Sistema
  1 0.310214 r24_a     R24
  2 0.428232 r24_a     R24
  3 0.460971 r24_a     R24
  4 0.482923 r24_a     R24
  5 0.492656 r24_a     R24
  6 0.515591 r24_a     R24
  &gt; tail(estacado)
            values   ind Sistema
  4999995 1.723030 g24_e     G24
  4999996 1.718444 g24_e     G24
  4999997 1.720745 g24_e     G24
  4999998 1.737137 g24_e     G24
  4999999 1.733106 g24_e     G24
  5000000 1.738487 g24_e     G24
  </code></pre>
  
  <p><a href="http://dl.dropbox.com/u/19202810/Rplot01.png" rel="nofollow noreferrer">http://dl.dropbox.com/u/19202810/Rplot01.png http://dl.dropbox.com/u/19202810/Rplot01.png</a> </p>
  
  <p>In the mentioned link, they modified the function <code>geom_boxplot</code>; I was hoping something more easy like <code>outlier.colour= alpha("black", 1/2)</code> but that gives the error:</p>
  
  <pre><code>Error in do.call("layer", list(mapping = mapping, data = data, stat = stat,  : 
    could not find function "alpha"
  </code></pre>
  
  <p>Any help is appreciated.</p>