title: Ternary plot and filled contour
body: <p>Users, I'd like to have some tips for a ternaryplot ("vcd").</p>
  
  <p>I have this dataframe:</p>
  
  <pre><code>a &lt;- c(0.1, 0.5, 0.5, 0.6, 0.2, 0, 0, 0.004166667, 0.45) 
  b &lt;- c(0.75,0.5,0,0.1,0.2,0.951612903,0.918103448,0.7875,0.45)
  c &lt;- c(0.15,0,0.5,0.3,0.6,0.048387097,0.081896552,0.208333333,0.1) 
  d &lt;- c(500,2324.90,2551.44,1244.50, 551.22,-644.20,-377.17,-100, 2493.04) 
  df &lt;- data.frame(a, b, c, d)
  </code></pre>
  
  <p>and I'm building a ternary plot:</p>
  
  <pre><code>ternaryplot(df[,1:3], df$d)
  </code></pre>
  
  <p>How can I map the continuous variable <code>d</code>, obtaining a result similar to this one?</p>
  
  <p><img src="https://i.stack.imgur.com/cmqRV.jpg" alt="enter image description here"></p>