title: Display nvd3 rChart on flexdashboard
body: <p>I'm trying to get an <code>nvd3</code> chart to render using <code>flexdashboard</code>.  Could anyone please point out what I'm doing wrong?  I've tried a few things, there as two examples of what I've tried below. </p>
  
  <p>Thanks for any help.</p>
  
  <p><em>flex.Rmd</em></p>
  
  <pre><code>---
  title: "Untitled"
  output: 
    flexdashboard::flex_dashboard:
      orientation: columns
      vertical_layout: fill
  runtime: shiny
  ---
  
  ```{r setup, include=FALSE}
  library(flexdashboard)
  library(rCharts)
  library(knitr)
  library(shiny)
  
  dat &lt;- data.frame(
    t = rep(0:23, each = 4), 
    var = rep(LETTERS[1:4], 4), 
    val = round(runif(4*24,0,50))
  )
  
  output$chart1 &lt;- renderChart({
   chrt1 &lt;- nPlot(val ~ t, group =  'var', data = dat, 
   type = 'stackedAreaChart', id = 'chart')
  
   return(chrt1)
  })
  
  chrt1 &lt;- nPlot(val ~ t, group =  'var', data = dat, 
   type = 'stackedAreaChart', id = 'chart')
  
  ```
  
  Column {data-width=650}
  -----------------------------------------------------------------------
  
  ### Chart A
  
  ```{r}
  # showOutput("chart1", "nvd3")
  
  renderChart({ chrt1$print("hi") })
  ```
  </code></pre>