title: rCharts (dimple) area chart with negative series
body: <p>I have the following problem; I am using the rCharts wrapper around dimple to produce an area chart. I realized when I add a series with negative values (v3), the chart series colour and the legend are different. Below is the following code;</p>
  
  <pre><code>library(rCharts)
  library(reshape2)
  seq(as.Date("2011-3-1"), by = "month", length.out = 12)
  df &lt;- data.frame(date= 2003:2014,
              v1 = runif(12,100,700), 
              v2 = runif(12,10,70) ,
              v3 = -1*runif(12,3,15))
  dat &lt;- melt(df,id="date")
  dat$date &lt;- as.character(dat$date)
  p &lt;- dPlot(value ~ date,
        groups = "variable",
        data = dat,
        type = "area")
  p$legend( x = 60, y = 10, width = 620, height = 20,
        horizontalAlign = "right")
  p
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/7g6wl.jpg" alt="enter image description here"></p>
  
  <p>I would appreciate it if you can help.</p>