title: nvd3 scatterPlot with rCharts in R: Vary size of points?
body: <p>I've been playing with rCharts and nvd3 for a bit of time now. Now I'm in a situation where I need a bubble chart, or at least a scatterplot where the size of the dots is dependent on a variable in the data. From <a href="http://nvd3.org/ghpages/scatter.html">this example</a>, it seems possible. The example on scatter charts in rCharts is:</p>
  
  <pre><code>library(rCharts)
  p1 &lt;- nPlot(mpg ~ wt, group = 'cyl', data = mtcars, type = 'scatterChart')
  p1$xAxis(axisLabel = 'Weight')
  p1
  </code></pre>
  
  <p>So I've tried setting size to, for example <code>gears</code>. But it doesn't change anything.</p>
  
  <pre><code>p2 &lt;- nPlot(mpg ~ wt, group = 'cyl', size = 'gear', data = mtcars, type = 'scatterChart')
  p2$xAxis(axisLabel = 'Weight')
  p2
  </code></pre>
  
  <p>Is it possible?</p>