title: how to set xAxis pointInterval(update: tickInterval) in highcharts
body: <p>I want to set the xAxis pointInterval in highstock.</p>
  
  <p>I'm formatting the xAxis:</p>
  
  <pre><code>xAxis: {
          type: 'datetime',
          labels: {
              formatter: function () {
                  var someDate = new Date(this.value);
                  return Myfunction(new Date(someDate));
              }
          }
      },
  </code></pre>
  
  <p>I have searched and found some ways but they were not worked for me! In order to set the pointInterval for a day( 24 * 3600 * 1000 // one day ) I set it when I was adding a series:</p>
  
  <pre><code>chart.addSeries({
                  name: my name,
                  data: my data,
                  id: my id,
                  type: 'spline',
                  pointStart:start date,
                  pointInterval: 24 * 3600 * 1000 // one day
              });
  </code></pre>
  
  <p>but It didn't work. so I tried to do sth else:</p>
  
  <pre><code>plotOptions: {
          spline: {
              pointStart: start date,
              pointInterval: 24 * 3600 * 1000 // one day
          }
      },
  </code></pre>
  
  <p>It also did not work.</p>
  
  <p>I've tested:</p>
  
  <pre><code>chart.xAxis[0].setCategories([data])
  </code></pre>
  
  <p>but this code makes the CPU working a lot and the browser crashes!</p>
  
  <p>Actually I've seen these examples. but when I try them they aren't useful!</p>
  
  <p><a href="http://jsfiddle.net/larsenmtl/SJSwt/1/" rel="nofollow">http://jsfiddle.net/larsenmtl/SJSwt/1/</a></p>
  
  <p><a href="http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/highcharts/xaxis/labels-overflow/" rel="nofollow">http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/highcharts/xaxis/labels-overflow/</a></p>
  
  <p>Please help me!</p>
  
  <p>Thank you</p>
  
  <p>UPDATE: my data is formatted like this:</p>
  
  <pre><code>series: [{
          data: [
              [Date.UTC(2010, 0, 1), 29.9],
              [Date.UTC(2010, 0, 2), 71.5],
              [Date.UTC(2010, 0, 3), 106.4],
  </code></pre>
  
  <p>e.g. jsfiddle.net/bahar_Agi/J6H7f</p>