title: highcharts xAxis pointStart issue
body: <p><a href="http://jsfiddle.net/whrTH/1/" rel="nofollow">http://jsfiddle.net/whrTH/1/</a><br>
  the only different</p>
  
  <pre><code> chart.series[0].update({
              pointStart: beginDate,
              pointInterval: 24 * 3600 * 1000 * 7,
              data: someData
          }, false);
  
   chart.series[0].update({
              pointStart: beginDate,
              pointInterval: 24 * 3600 * 1000 * 1,
              data: someData
          }, false);
  </code></pre>
  
  <p>when i click the day button, the code work well,  when i click the week button, it works also, However  the style of label show on xAxis is not what i want, what i want is every point has its label show on xAxis, just like when i click the day button. any help  </p>