title: geom_violin() - figure border is curved in a strange way
body: <p>I'm investigating data with smooth and almost convex distribution of integer values. But when I print it with <code>geom_violin()</code> I got strange shape like there are very rare intermediate non-integer values. How to avoid these "bottlenecks" on the plot near <code>y = 1.5</code> &amp; <code>y = 2.5</code> and what do they mean?</p>
  
  <pre><code>require( ggplot2)
  df &lt;- data.frame( x = 1, y = c( rep( 1, 100), rep( 2, 25), rep( 3, 5)))
  ggplot( data = df, aes( x, y)) + geom_violin( )
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Nsqez.jpg" alt="distribution bottlenecks plot"></p>