title: plotly have trace without data in legend
body: <p><a href="https://stackoverflow.com/questions/33027334/categorical-axis-order-in-plotly-js">As a follow on to a previous question</a></p>
  
  <p>Suppose I want to show a set of bars, where each bar could be one of the choices: A, B, or C, each which has a different color. I would like to show the bars, but also if none the bars fall into a particular category still have that category show up in the legend. Unfortunately the categories without colors seem to get dropped. Notice in this example how the category labeled 'C' which should be blue is dropped from the legend:</p>
  
  <pre><code>data = [
  {
  x: [1, 3, 4],
  y: [20, 14, 23],
  type: 'bar',
  name: 'A',
  marker: {color: '#00FFFF'}},
  {
  x: [2, 5, 6],
  y: [8, 6, 2],
  type: 'bar',
  name: 'B',
  marker: {color: '#FF00FF'}},
  {
  x: [],
  y: [],
  type: 'bar',
  name: 'C'
  marker: {color: '#FF0000'}}]
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/LFXbz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LFXbz.png" alt="Category C is not shown"></a></p>
  
  <p>How would I be able to make sure C (or any color without data) always shows up?</p>