title: Highcharts datetime axis labels according to data point groups
body: <p>Is it possible to have labels on datetime xAxis that correspond to dataGrouping?
  For example I have <code>dataGrouping</code> such that data are grouped into days / weeks / months / year:</p>
  
  <pre><code>dataGrouping : {
    groupPixelWidth: 250,
    units : [
      ['day', [1]], ['week', [1]], ['month', [1]], ['year', [1]]
    ]
  }
  </code></pre>
  
  <p><a href="http://jsfiddle.net/CHR7F/2/" rel="nofollow">http://jsfiddle.net/CHR7F/2/</a></p>
  
  <p>There is plenty of options in the highcharts api, but no one seems to achieve the same result as dataGrouping.</p>
  
  <p>The most promising was:</p>
  
  <pre><code>xAxis: {
     labels: {
         step: 1
     }
  }
  </code></pre>
  
  <p>but still there are two weeks between ticks while data is grouped by months.</p>