title: Highcharts chart with 'datetime' xAxis - use categories on drilldown
body: <p>Is there a way to have 'datetime' for the xAxis type on the main series, but then when a series is clicked on, have the drilldown use categories for that time?  </p>
  
  <p>In this jsfiddle example (<a href="http://jsfiddle.net/kadams/3e3xqv7e/" rel="noreferrer">http://jsfiddle.net/kadams/3e3xqv7e/</a>), you can see that when 'category' is used as the xAxis type, the drilldown data correctly uses the drilldown series names 'A', 'B', and 'C' on the xAxis.  But when the xAxis type is changed to 'datetime', and and the millisecond time is used for the 'x' value in place of a name for the main series, the categories on the drilldown don't show 'A', 'B', or 'C' anymore.  Just meaningless dates.</p>
  
  <p><strong>UPDATE for clarification</strong> - I would prefer to use the 'datetime' type instead of 'category' type with values formatted as dates, because Highcharts will throw the 'too many ticks' error when the x-axis is big: <a href="http://www.highcharts.com/errors/19" rel="noreferrer">http://www.highcharts.com/errors/19</a>.  I give the 'category' type example in the fiddle below simply to demonstrate that 'A', 'B', 'C' do show properly when the type is not 'datetime'.</p>
  
  <pre><code>$(function () {
      $('#container').highcharts({
          chart: {
              type: 'column',
          },
          xAxis: {
              type: 'category',
              //  type: 'datetime',
              dateTimeLabelFormats: {
                  hour: '%l:%M %p'
              }
          },
          legend: {
              enabled: false
          },
          series: [{
              name: 'Total',
              colorByPoint: true,
              data: [{
                  y: 8,
                  drilldown: 'Bob',
                  name: 'Bob', //used with 'category' xAxis type
                  x: 1420700400000 //used with 'datetime' xAxis type
              }]
          }],
          drilldown: {
              series: [{
                  id: 'Bob',
                  name: 'Bob',
                  data: [{
                      name: 'A',
                      y: 3
                  }, {
                      name: 'B',
                      y: 3
                  }, {
                      name: 'C',
                      y: 2
                  }]
              }]
          }
      });
  });
  </code></pre>