title: ggvis side-by-side barchart grouped by second variable
body: <p>I am in process of migrating from Excel to ggvis for data analysis. For a typical grouped bar chart with two variables, however I have difficulty to <strong>plot</strong> <strong>bar chart side-by-side instead of stacked</strong>.</p>
  
  <p>The following data has four steps A, B, C, D with "ratio" data from two features cc, ca. My attempt is to plot the ratio from cc and ca features side-by-side. However, the default plot with stack the two data together. Check the ggvis vignetts has an option to set stack =FALSE. But it would overlap the other feature. </p>
  
  <p><strong>Is there an option in ggvis to do things like "geom_bar(position="dodge")" in ggplot?</strong> </p>
  
  <pre><code>library(ggvis)
  steps &lt;-c("A","B","C","D","A","B","C","D")
  ratio &lt;-c(1.1,1.5,1.7,1.4,1.5,1.7,1.4,1.9)
  feature &lt;-c("cc","cc","cc","cc","ca","ca","ca","ca")
  shrink &lt;- data.frame(steps,ratio,feature)
  shrink %&gt;% ggvis(x= ~steps, y= ~ratio, fill = ~feature) %&gt;% layer_bars()
  </code></pre>