title: Legend for multi surface plot with specific colors
body: <p>Take the example for <code>plot_ly</code> function:</p>
  <pre><code>library(&quot;plotly&quot;)
  z &lt;- c(
    c(8.83,8.89,8.81,8.87,8.9,8.87),
    c(8.89,8.94,8.85,8.94,8.96,8.92),
    c(8.84,8.9,8.82,8.92,8.93,8.91),
    c(8.79,8.85,8.79,8.9,8.94,8.92),
    c(8.79,8.88,8.81,8.9,8.95,8.92),
    c(8.8,8.82,8.78,8.91,8.94,8.92),
    c(8.75,8.78,8.77,8.91,8.95,8.92),
    c(8.8,8.8,8.77,8.91,8.95,8.94),
    c(8.74,8.81,8.76,8.93,8.98,8.99),
    c(8.89,8.99,8.92,9.1,9.13,9.11),
    c(8.97,8.97,8.91,9.09,9.11,9.11),
    c(9.04,9.08,9.05,9.25,9.28,9.27),
    c(9,9.01,9,9.2,9.23,9.2),
    c(8.99,8.99,8.98,9.18,9.2,9.19),
    c(8.93,8.97,8.97,9.18,9.2,9.18)
  )
  dim(z) &lt;- c(15,6)
  z2 &lt;- z + 1
  
  fig &lt;- plot_ly(showscale = FALSE)
  fig &lt;- fig %&gt;% add_surface(z = ~z, colorscale = list(c(0,1),c(&quot;rgb(107,184,255)&quot;,&quot;rgb(0,90,124)&quot;)))
  fig &lt;- fig %&gt;% add_surface(z = ~z2, colorscale = list(c(0,1),c(&quot;rgb(255,107,184)&quot;,&quot;rgb(128,0,64)&quot;)))
  
  </code></pre>
  <p>How can I add legends to this 3d plot?</p>