title: Angular highcharts problem showing week number
body: <p>I am running into a problem with highcharts in my Angular application, that i simple can´t resolve. </p>
  
  <p>I'm trying to show the week number on the xAxis with the type: 'datetime' as shown below. I found this <a href="http://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/global/dateformats" rel="nofollow noreferrer">JSFiddle</a> thats should solve the problem, but when iam trying to use this in my code i will get this error <code>Cannot assign to 'dateFormats' because it is a constant or a read-only property.</code> At the bottom of this description I have included the dateFormats description, shouldn't it be possible to do this? </p>
  
  <p>What am I overlooking here :)</p>
  
  <p><strong>ERROR OCCURS ON LINE 1 IN THE FOLLOWING CODE:</strong></p>
  
  <pre><code>    Highcharts.dateFormats = {
          W: function (timestamp) {
              console.log('timestamp', timestamp);
              return 45;
          }
      };
  
      Highcharts.setOptions({
          xAxis = {
              type: 'datetime',
              labels: {
                  format: '{value:Week %W/%Y}'
              },
              minTickInterval: 1000 * 3600 * 24 * 7,
              minRange: 1000 * 3600 * 24 * 7
          };
      });
  </code></pre>
  
  <p><strong>FROM HIGHCHARTS FILE</strong></p>
  
  <pre><code>/**
   * A hook for defining additional date format specifiers. New specifiers are defined as key-value pairs by using the
   * specifier as key, and a function which takes the timestamp as value. This function returns the formatted portion
   * of the date.
   */
  dateFormats: DateFormatSpecifiers;
  </code></pre>