title: HighChart Drilldown axis automatically show the differences instead of actual label
body: <p><a href="http://jsfiddle.net/sohxow92/" rel="nofollow noreferrer">http://jsfiddle.net/sohxow92/</a></p>
  
  <p>The <code>y-axis</code> after clicking on <code>drilldown</code> automatically show the short label, <code>try 1-2h</code>, and take a look at <code>y-axis</code> you will see, starting from <code>68</code>, it shows <code>8, 12</code> and so on.
  What config of <code>highchart</code> do I need to modify to make this show the original label?
  Following is part of the drilldown object.</p>
  
  <pre><code>"drilldown": {
      "xAxis": {
      "type": "category",
      "labels": {
        "overflow": "justify"
      }
    },
    "yAxis": {
      "labels": {
        "overflow": "justify"
      },
      "title": {
        "text": "Visits"
      }
    },
    "activeDataLabelStyle": {
      "textDecoration": "none"
    },
    "activeAxisLabelStyle": {
      "textDecoration": "none"
    },
  }
  </code></pre>
  
  <p>Thanks in advance.</p>