title: Seaborn - compress violinplot for zero count categories
body: <p>Is there a simple way to ignore zero count categories when laying out a violinplot. In the example below, there are no cases of 'Yes:Red' and 'No:Green' but the violinplot still plots the "missing" categories. I can see why this should be the default behavior, but is there some way to change the factors used in the hue to suppress this and remove the whitespace?</p>
  
  <pre><code>df = pd.DataFrame(
      {'Success': 50 * ['Yes'] + 50 * ['No'], 
       'Category': 25 * ['Green'] + 25 * ['Blue'] + 25 * ['Green'] + 25 * ['Red'],
       'value': np.random.randint(1, 25, 100)}
  )
  sns.violinplot(x='Success', y='value', hue='Category', data=df)
  plt.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/b5kYY.png" rel="noreferrer"><img src="https://i.stack.imgur.com/b5kYY.png" alt="enter image description here"></a></p>
  
  <p>Thanks in advance.</p>