title: Synchronized charts example with Highstock crashes with "Cannot read property 'category' of undefined"
body: <p>I want the functionality of the "Synchronized charts" example, but with Highstock. But when trying to accomplish this, I get "highstock.src.js:9991 Uncaught TypeError: Cannot read property 'category' of undefined"</p>
  
  <p>This also holds directly for the example: <a href="http://www.highcharts.com/demo/synchronized-charts" rel="nofollow">http://www.highcharts.com/demo/synchronized-charts</a> doesn't work when converted to Highstock: <a href="http://jsfiddle.net/9gq47g0w/" rel="nofollow">http://jsfiddle.net/9gq47g0w/</a></p>
  
  <p>(Since StackOverflow demands me to post some code along with the fiddle, here's from Highstock, noting the point where it crashes with **):</p>
  
  <pre><code>    /**
       * Refresh the tooltip's text and position.
       * @param {Object} point
       */
      refresh: function (point, mouseEvent) {
          ...
          // shared tooltip, array is sent over
          if (shared &amp;&amp; !(point.series &amp;&amp; point.series.noSharedTooltip)) {
              ...
              textConfig = {
                  x: ** point[0].category, ** &lt;- here!
                  y: point[0].y
              };
             ...
          }
          ...
      },
  </code></pre>