title: How to specify ggplot2 boxplot fill colour for continuous data?
body: <p>I want to plot a ggplot2 boxplot using all columns of a data.frame, and I want to reorder the columns by the median for each column, rotate the x-axis labels, and <strong>fill each box with the colour corresponding to the same median</strong>. I can't figure out how to do the last part. There are plenty of examples where the fill colour corresponds to a <em>factor</em> variable, but I haven't seen a clear example of using a <em>continuous</em> variable to control fill colour. (The reason I'm trying to do this is that the resultant plot will provide context for a force-directed network graph with nodes that will be colour-coded in the same way as the boxplot -- the colour will then provide a mapping between the two plots.) It would be nice if I could re-use the value-to-colour mapping for later plots so that colours are consistent between plots. So, for example, the box corresponding to the column variable with a high median value will have a colour that denotes this mapping and matches perfectly the colour for the same column variable in other plots (such as the corresponding node in a force-directed network graph).</p>
  
  <p>So far, I have something like this:</p>
  
  <pre><code># Melt the data.frame:
  DT.m &lt;- melt(results, id.vars = NULL) # using reshape2
  # I can now make a boxplot for every column in the data.frame:
  g &lt;- ggplot(DT.m, aes(x = reorder(variable, value, FUN=median), y = value)) +
  theme(axis.text.x = element_text(angle = 90, hjust = 1)) +
  stat_summary(fun.y=mean, colour="darkred", geom="point") +
  geom_boxplot(???, alpha=0.5)
  </code></pre>
  
  <p>The colour fill information is what I'm stuck on. "value" is a continuous variable in the range [0,1] and there are 55 columns in my data.frame. Various approaches I've tried seem to result in the boxes being split vertically down the middle, and I haven't got any further. Any ideas?</p>