title: R plotly sankey nodes opacity
body: <p>The default sankey chart has links that are grey and transparent so that you can see the overlapping links. I am trying add color to the links in the example Sankey chart below; is there a way to make the links partially transparent so I can see overlapping links?</p>
  
  <pre><code>p &lt;- plot_ly(
    type = "sankey",
    orientation = "h",
  
    node = list(
      label = c("A1", "A2", "B1", "B2", "C1", "C2"),
      color = c("blue", "blue", "blue", "blue", "blue", "blue"),
      pad = 15,
      thickness = 20,
      line = list(
        color = "black",
        width = 0.5
      )
    ),
  
    link = list(
      source = c(0,1,0,2,3,3),
      target = c(2,3,3,4,4,5),
      value =  c(8,4,2,8,4,2),
      color = c("blue", "red", "green", "yellow","blue", "red"),
      opacity = c(0.5, 0.5, 0.5, 0.5, 0.5, 0.5),
      alpha = 0.5
    )
  ) %&gt;% 
    layout(
      title = "Basic Sankey Diagram",
      font = list(
        size = 10
      )
    )
  </code></pre>