title: Display percentage change in the title of Highcharts timeseries
body: <p>I have a use case where I need to display the percentage change in value of a time series chart in the chart title.</p>
  
  <p>So far, I have tried getting the first and last element of the series, but this only works when all the data is visible. I need a way to account for the zoom buttons and the range selector elements.</p>
  
  <p>Any suggestions on how to obtain the first and last axis y values of the current date range?</p>
  
  <pre><code>$(function() {
      $.getJSON('http://www.highcharts.com/samples/data/jsonp.php?filename=aapl-c.json&amp;callback=?', function(data) {
          // Create the chart
          $('#container').highcharts('StockChart', {
              rangeSelector : {
                  selected : 1,
              },
  
              title : {
                  text : 'AAPL price change +5%' // how to get the %?
              },
  
              series : [{
                  name : 'AAPL',
                  data : data,
                  tooltip: {
                  valueDecimals: 2
                  }
              }], 
          });
      });
  });
  </code></pre>
  
  <p>Edit: I have tried to adapt another example to my needs: <a href="http://jsfiddle.net/F8MjD/5/" rel="nofollow">http://jsfiddle.net/F8MjD/5/</a></p>
  
  <p>Example used: <a href="http://jsfiddle.net/NsD3s/7/" rel="nofollow">http://jsfiddle.net/NsD3s/7/</a></p>
  
  <p>But still doesn't work, even though it looks exactly the same to me?</p>