title: add extra legend to plot
body: <p>Hi can I add an additional legend to a ggplot.
  Like
  the following code</p>
  
  <pre><code>d &lt;- melt(as.matrix(data.frame(y1=1/(1:10),y2=1/(10:1))))
  ggplot(d, aes(x=Var1, y=value,fill=Var2)) + geom_bar(stat="identity",position='dodge')
  </code></pre>
  
  <p>This generates a nice legend containing the name of my dataframe.
  But is it possible to put in an extralegend, that contains some extra information generated from the data.</p>
  
  <p>In the standard R, I would add the additional legend like</p>
  
  <pre><code>d&lt;-data.frame(y1=1/(1:10),y2=2*1/(10:1))
  barplot(t(d),beside=T)
  legend("top",paste("sums:",apply(d,2,sum)))
  </code></pre>
  
  <p>Thanks</p>