title: Adding gap between specific bars in a stacked bar chart
body: <p>I have two groups of stacked bars in a chart and I would like to group them visually</p>
  
  <p>So how could I add extra vertical gap between specific bars in a stacked bar chart or separate the groups in some other way? </p>
  
  <p>Example:
  <a href="http://jsfiddle.net/KrTbz/41/" rel="nofollow">http://jsfiddle.net/KrTbz/41/</a></p>
  
  <p>I would like to add extra vertical space between bars 4 and A</p>
  
  <pre><code> new Highcharts.Chart({
          chart: {
              renderTo: 'container',
              type: 'bar',
          },
          title: {
              text: 'Grouping test',
          },
          xAxis: {
              categories: ['1', '2', '3', '4', 'A', 'B', 'C', ]
          },
          plotOptions: {
              series: {
                  stacking: 'percent'
              }
          },
          series: [{
              name: 'Alpha',
              data: [31, 27, 23, 22, 22, 20, 21]
          }, {
              name: 'Omega',
              data: [5, 6, 7, 5, 6, 6, 5, ]
          }, {
              name: 'Gamma',
              data: [60, 62, 67, 69, 68, 70, 71]
          }]
      });
  });
  </code></pre>