title: xDateFormat string doesn't apply to large amounts of data
body: <p>I have an issue that <strong>I have been able to replicate on JFiddle</strong>.  Link: <a href="http://jsfiddle.net/h5sSR/" rel="nofollow">http://jsfiddle.net/h5sSR/</a></p>
  
  <pre><code>$(function() {
  var chart = new Highcharts.StockChart({
  
      chart: {
          renderTo: 'container'
      },
  
      tooltip: {
          backgroundColor: {
              linearGradient: {
                  x1: 0, 
                  y1: 0, 
                  x2: 0, 
                  y2: 1
              },
              stops: [
                  [0, 'white'],
                  [1, '#EEE']
              ]
          },
          xDateFormat: '%m/%e/%y %H:%M',
          borderColor: 'gray',
          borderWidth: 1
      },
  
      rangeSelector: {
          selected: 1
      },
  
      series: [{
          name: 'USD to EUR',
          data: usdeur
      }]
  });
  });
  </code></pre>
  
  <p><strong>The Problem</strong>: I need the xDateFormat attribute to be applied under ToolTip at all times (<em>month</em> / <em>day</em> / <em>year</em>  <em>hour</em> : <em>minute</em>).  It works perfectly when you have a small amount of data selected.  It uses a different format when you have a large amount of data selected to view.  </p>
  
  <p><strong>The Example</strong>: Try it on the JFiddle (link above).  It initializes to a small amount of data visible and when you hover, you will see the Date like so: 12/3/12 0:0.  That's perfect.  Now drag the navigator to the beginning and hover on the graph, the DateFormat changes to (Week from <em>Day of Week</em>, <em>Month Day</em>, <em>Year</em>).</p>
  
  <p><strong>How to Solve</strong>: Why is this happening and how can I apply the xDateFormat attribute all the time regardless of how much data is being displayed?</p>