title: Highcharts - second xAxis labels are not displayed
body: <p>I have a bar chart with two sets of data. Inspired by the variant from <a href="http://www.highcharts.com/demo/bar-negative-stack" rel="nofollow">http://www.highcharts.com/demo/bar-negative-stack</a>, the only difference is that the two sets of data have different set of categories. The second xAxis now don't use the <code>linkedTo</code> attribute and I wonder why the labels are not displayed:</p>
  
  <p><a href="http://jsfiddle.net/UTPR7/1/" rel="nofollow">http://jsfiddle.net/UTPR7/1/</a></p>
  
  <pre><code>$(document).ready(function() {
      $('#container').highcharts({
          chart: {
              type: 'bar'
          },
          xAxis: [{
              categories: ['A','B','C']
          }, {
              opposite: true,
              categories: ['D','E','F']
          }],
  
          plotOptions: {
              series: {
                  stacking: 'normal'
              }
          },
  
          series: [{
              data: [-1746181, -1884428, -2089758]
          }, {
              data: [1656154, 1787564, 1981671]
          }]
      });
  });
  </code></pre>