title: Gradient color in links of sankey chart using networkD3
body: <p>I am using <a href="https://christophergandrud.github.io/networkD3/#sankey" rel="nofollow noreferrer">networkD3</a> to create sankey diagrams in R. I am using something like below to generate the diagrams:</p>
  
  <pre><code>sankeyNetwork(Links = data$links, Nodes = data$nodes, 
            Source = "source", Target = "target", Value = "value", NodeID = "name",
            units = "Questions", fontSize = 12, nodeWidth = 10, 
            colourScale = "d3.scale.category10()",
            LinkGroup = "type",             #&lt;--assign colors to links
            width = 1000, height = 600)
  </code></pre>
  
  <p>I am using <code>LinkGroup = "type"</code> to color the links, where the <code>type</code> is basically the source node's name. This makes the link color same as source node color. This is working pretty well.</p>
  
  <p>I was wondering if there is anyway a gradient of color can be created from sources to targets for the associated links?</p>