title: Split violin plot with ggplot2 with quantiles
body: <p>In order to plot half densities, I am using the function described in this post: <a href="https://stackoverflow.com/questions/35717353/split-violin-plot-with-ggplot2">Split violin plot with ggplot2</a></p>
  
  <p>However, when I want to draw the quantiles on the densities, like on a normal <code>geom_violin()</code> or <code>geom_boxplot()</code>, I obtain an error message.</p>
  
  <p>I would also be interested in adding the number of observations above each half density.</p>
  
  <p>Here is an example of what I would like to obtain:</p>
  
  <pre><code>data("diamonds")
  library(ggplot2)
  
  # Function described in a previous post
  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$y))) {
      stopifnot(all(draw_quantiles &gt;= 0), all(draw_quantiles &lt;= 
                                                1))
      quantiles &lt;- create_quantile_segment_frame(data, draw_quantiles)
      aesthetics &lt;- data[rep(1, nrow(quantiles)), setdiff(names(data), c("x", "y")), drop = FALSE]
      aesthetics$alpha &lt;- rep(1, nrow(quantiles))
      both &lt;- cbind(quantiles, aesthetics)
      quantile_grob &lt;- GeomPath$draw_panel(both, ...)
      ggplot2:::ggname("geom_split_violin", grobTree(GeomPolygon$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, ...))
  }
  
  tmp &lt;- diamonds[which(diamonds$cut %in% c("Fair", "Good")), ]
  
  # Obtained plot
  ggplot(tmp, aes(as.factor(color), carat, fill = cut)) +
    geom_split_violin()
  
  # Error due to internal functions (interleave, ...)
  ggplot(tmp, aes(as.factor(color), carat, fill = cut)) +
    geom_split_violin(draw_quantiles = 0.5)
  
  # Function to return number of observation
  give_n = function(x, y_up = y_upper) {
    data.frame(y = y_up * 1.06,
               label = paste("n =", length(x))
    )
  }
  
  # Code to add number of observations above each half density
  new_plot = given_plot +
    # Give back only length of data
    stat_summary(fun.data = give_n, aes(x = as.factor(variable)), geom = "text")
  </code></pre>