title: Customizing ggplot Legend
body: <p>Say I have a dataset like this</p>
  
  <pre><code>  X1 X2 X3 X4 X5 X6
  1  3  4  5  6  7  5
  2  6  7  6  3  1  4
  3  8  7  9  1  2  4
  </code></pre>
  
  <p>and code like this </p>
  
  <pre><code>ggplot(data=melt(vecDF), aes(x = variable, y=value)) + geom_violin(aes(fill=variable))+
  scale_fill_manual(values=c("red","red","blue","blue","green","green"))+
   ggtitle("Coverage")+xlab("Location")+ylab("Value")
  </code></pre>
  
  <p>How do I alter it so that the I get a legend with simply three boxes instead of 6. A red, blue and green one with custom names. I can add a group column to the dataframe but as soon as I melt it it becomes useless for grouping the columns together. </p>