title: D3 Group label for xAxis ticks
body: <p>I have a similar dataset:</p>
  
  <pre><code>[
    {
      date: '2019-01-01',
      value: 100,
      group: 1
    },
    {
      date: '2019-01-02',
      value: 200,
      group: 1
    },
    {
      date: '2019-01-03',
      value: 200,
      group: 2
    },
    {
      date: '2019-01-04',
      value: 200,
      group: 3
    },
    {
      date: '2019-01-05',
      value: 100,
      group: 3
    },
    {
      date: '2019-01-06',
      value: 250,
      group: 3
    }
  ]
  </code></pre>
  
  <p>I've create a linear chart with yAxis - <code>value</code>, and xAxis - <code>date</code>.</p>
  
  <p>How can I implement additional label for xAxis by <code>group</code> field value, like I show on the pic?</p>
  
  <p><a href="https://i.stack.imgur.com/CamJZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CamJZ.png" alt="enter image description here"></a></p>