title: R violin plot overlay 2 dataframes
body: <p>Say you have two dataframes </p>
  
  <pre><code>M1 &lt;- data.frame(sample(1:3, 500, replace = TRUE), ncol = 5)
  M2 &lt;- data.frame(sample(1:3, 500, replace = TRUE), ncol = 5)
  </code></pre>
  
  <p>and I want to overlay them as violin plots as seen here:
  <a href="https://stackoverflow.com/questions/19483074/overlay-violin-plots-ggplot2">Overlay violin plots ggplot2</a></p>
  
  <p>but I have 2 dataframes like above (but bigger) not one with 3 columns as in the example above</p>
  
  <p>I have tried the advice using melt as seen here:
  <a href="https://stackoverflow.com/questions/21960614/violin-plot-of-a-data-frame">Violin plot of a data frame</a></p>
  
  <p>but I cant get it to overlay two dataframes</p>
  
  <p>help is much appreciated: </p>