title: Highcharts datagrouping issue
body: <p>I am facing a strange problem with highcharts(highstock) datagrouping. I want to show grouped data in week or month and at the same time zoom the chart to a specific period (I am using chart.yAxis[0].min = ). When the width of the overall chart is high and when I set the the zoom level to a short period (say a few months), data is not grouped to the period I specify.</p>
  
  <p>Refer to the fiddle link below. I have specified datagrouping as 'month' (<code>{units:[['month',[1]]], enabled:true}</code>) and yAxis[0].min as 29 Jul 2010. You will observe that data is not grouped in months.
  Now try to reduce the width jsfiddle result panel by dragging the divider. When the size of the result area (or the chart) is reduced, datagrouping starts working.</p>
  
  <p><a href="http://jsfiddle.net/amit657/of1mv8yv/2/" rel="nofollow">http://jsfiddle.net/amit657/of1mv8yv/2/</a></p>
  
  <p>Any idea how can I force datagrouping and at the same zoom the chart to show data for a specific period?</p>