title: aes() and aes_string() produce different behaviors in split_violin_plot
body: <p>I am trying to use a function from this <a href="https://stackoverflow.com/questions/35717353/split-violin-plot-with-ggplot2">post</a> to produce a split violin plot. </p>
  
  <p>Here's the code:</p>
  
  <pre><code>GeomSplitViolin &lt;- ggproto("GeomSplitViolin", GeomViolin, draw_group = function(self, data, ..., draw_quantiles = NULL){
    data &lt;- transform(data, xminv = x - violinwidth * (x - xmin), xmaxv = x + violinwidth * (xmax - x))
    grp &lt;- data[1,'group']
    newdata &lt;- plyr::arrange(transform(data, x = if(grp%%2==1) xminv else xmaxv), if(grp%%2==1) y else -y)
    newdata &lt;- rbind(newdata[1, ], newdata, newdata[nrow(newdata), ], newdata[1, ])
    newdata[c(1,nrow(newdata)-1,nrow(newdata)), 'x'] &lt;- round(newdata[1, 'x']) 
    if (length(draw_quantiles) &gt; 0 &amp; !scales::zero_range(range(data$y))) {
      stopifnot(all(draw_quantiles &gt;= 0), all(draw_quantiles &lt;= 
                                                1))
      quantiles &lt;- ggplot2:::create_quantile_segment_frame(data, draw_quantiles)
      aesthetics &lt;- data[rep(1, nrow(quantiles)), setdiff(names(data), c("x", "y")), drop = FALSE]
      aesthetics$alpha &lt;- rep(1, nrow(quantiles))
      both &lt;- cbind(quantiles, aesthetics)
      quantile_grob &lt;- GeomPath$draw_panel(both, ...)
      ggplot2:::ggname("geom_split_violin", grid::grobTree(GeomPolygon$draw_panel(newdata, ...), quantile_grob))
    }
    else {
      ggplot2:::ggname("geom_split_violin", GeomPolygon$draw_panel(newdata, ...))
    }
  })
  
  geom_split_violin &lt;- function (mapping = NULL, data = NULL, stat = "ydensity", position = "identity", ..., draw_quantiles = NULL, trim = TRUE, scale = "area", na.rm = FALSE, show.legend = NA, inherit.aes = TRUE) {
    layer(data = data, mapping = mapping, stat = stat, geom = GeomSplitViolin, position = position, show.legend = show.legend, inherit.aes = inherit.aes, params = list(trim = trim, scale = scale, draw_quantiles = draw_quantiles, na.rm = na.rm, ...))
  }
  </code></pre>
  
  <p>Here's an example dataset:</p>
  
  <pre><code>set.seed(20160229)
  my_data = data.frame(
    y=c(rnorm(1000), rnorm(1000, 0.5), rnorm(1000, 1), rnorm(1000, 1.5)),
    x=c(rep('a', 2000), rep('b', 2000)),
    m=c(rep('i', 1000), rep('j', 2000), rep('i', 1000))
  )
  </code></pre>
  
  <p>Use <code>aes()</code> in the plot function, we could get the normal plot</p>
  
  <pre><code>ggplot(my_data, aes(x, y, fill=m)) + geom_split_violin()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/J4Xbz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/J4Xbz.png" alt="enter image description here"></a></p>
  
  <p>But if use <code>aes_string()</code>, I got a weird plot where two split violin on top of each other. </p>
  
  <pre><code>ggplot(my_data, aes_string(x='x', y='y', fill='m')) + geom_split_violin()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/jtarw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jtarw.png" alt="enter image description here"></a></p>
  
  <p>I do not have a good understanding of how <code>aes</code> vs. <code>aes_string</code> might lead to this difference. Hope someone could give me a hint. Thanks a lot!</p>