title: HighChart: Add xaxis dynamically doesn't consider what type I give it
body: <p>I'm trying to add a xaxis dynamically using:</p>
  
  <pre><code>         $('#highchartcontainer').highcharts().addAxis({
                  id: i,
                  title: {
                      text: tag.name,
                      style: {
                          color: color
                      }
                  },
                  xAxis: {
                      type: 'datetime',
                  },
                  lineWidth: 1,
                  lineColor: color}, true);
  </code></pre>
  
  <p>ref: <a href="http://api.highcharts.com/highcharts#Chart.addAxis" rel="nofollow">http://api.highcharts.com/highcharts#Chart.addAxis</a></p>
  
  <p>But when it starts to draw I get this strange number as shown in the picture:
  <a href="https://imgur.com/HXjYPdV" rel="nofollow">https://imgur.com/HXjYPdV</a></p>
  
  <p>How do I get the newly added xaxis to be datetime?</p>