title: addControls for a pieChart using nPlot() in rChart
body: <p>Is addControls() compatible with type=pieChart using nPlot()?<br>
  The pie chart disappears as soon as I try to add controls.</p>
  
  <p>Im attempting the following: </p>
  
  <pre><code>Sp = c("a", "b", "c", "d") 
  A = c(10, 4, 3, 0) 
  B = c(3, 5, 22, 4) 
  C = c(1, 0 ,10, 8)
  df = data.frame(Sp, A, B, C)
  
  p1 &lt;- nPlot(x = "Sp", y ="A" ,
          color = 'Sp', data = df, type = 'pieChart')
  p1$addControls('y', 'A', values=c('A','B','C'))
  p1
  </code></pre>
  
  <p>It works perfectly if I use multiBarHorizontalChart instead.  </p>
  
  <p>Thank you!  </p>