title: ggplot violin plot, specify different colours by group?
body: <p>I have a matrix of 9 columns and I want to create a violin plot using <code>ggplot2</code>. I would like to have different colours for groups of three columns, basically increasing order of "grayness". How can I do this?</p>
  
  <p>I have tried imputing lists of colours on the option "fill=" but it does not work.  See my example below. At the moment, it indicates "gray80", but I want to be able to specify the colour for each violin plot, in order to be able to specify the colour for groups of 3.</p>
  
  <pre><code>library(ggplot2)
  dat &lt;- matrix(rnorm(100*9),ncol=9)
  
  # Violin plots for columns
  mat &lt;- reshape2::melt(data.frame(dat), id.vars = NULL)
  pp &lt;- ggplot(mat, aes(x = variable, y = value)) + geom_violin(scale="width",adjust = 1,width = 0.5,fill = "gray80")
  pp
  </code></pre>