title: What exactly is the z argument in plot_ly?
body: <p>I have three variables: <code>x</code>,<code>y</code>, and <code>z</code> and I want to produce a surface plot. </p>
  
  <pre><code>z&lt;-runif(50,0,1) 
  y&lt;-runif(50,1,2)
  x&lt;-runif(50,3,6)
  plot_ly(x = ~x, y = ~y, z= ~z) %&gt;% add_surface()
  </code></pre>
  
  <p>I get the following error</p>
  
  <p><code>Error: `z` must be a numeric matrix</code></p>
  
  <p>What exactly does <code>z</code> represent if not the variable corresponding to the vertical axis? I have seen the Volcano example where they use the matrix to generate that plot, but I still am not sure what that z matrix represents in that example either. </p>
  
  <p>What I would like is for someone to plot an easy to understand 3D function like <code>z=f(x,y) = x^2 + y^2</code> using the <code>surface</code> functionality in <code>plot_ly</code> just so I can understand how to generate a plot based on three variables. </p>