title: Categorical axes in plotly heatmapgl
body: <p>Hi I've some problems with heatmapgl plot (but with heatmap I get the same) of plotly package. 
  I've a distance matrix between a series of elements. I would like to plot a heatmap with names of elements on axes and possibly also on hover text labels. </p>
  
  <pre><code>library(plotly)
  values=c(22.63409, 323.93780, 366.31498, 467.94524, 461.93251, 643.61306, 665.03471, 482.49047, 500.06208, 662.71600, 673.29995, 567.99427, 558.41419, 632.86704, 615.55000, 508.01659, 462.50232, 411.39160, 399.00598, 286.55681, 268.69871) 
      z_matrix=matrix(NA,7,7)
      z_matrix[lower.tri(z_matrix, diag=FALSE)] &lt;- values
      elements_names=c("ATHOTPlun04", "ATHOTPlun10", "ATHOTPlun16", "ATHOTPlun22", "ATHOTPmar04", "ATHOTPmar10", "ATHOTPmar16")
  q &lt;- plot_ly(
      x=elements_names,
      y=elements_names,
      z = z_matrix, type = "heatmapgl") %&gt;%
      layout(title = 'Distances for ATHOTP route')
  q
  </code></pre>
  
  <p>Although this is what I get when I run the code:
  <a href="https://i.stack.imgur.com/Mcuea.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Mcuea.png" alt="enter image description here"></a></p>
  
  <p>I tried also to set axes type to "category" in layout options but nothing seems to change. What am I missing? Thank you!</p>