title: can't use strings instead of integers in area charts in morris charts library
body: <p><a href="http://jsbin.com/uzosiq/258/embed?javascript,live">Bar Chart Example</a> </p>
  
  <pre><code>Morris.Bar({
    element: 'bar-example',
    data: [
      { y: 'dddd', a: 100, b: 90 },
      { y: '2007', a: 75,  b: 65 },
      { y: '2008', a: 50,  b: 40 },
      { y: '2009', a: 75,  b: 65 },
      { y: '2010', a: 50,  b: 40 },
      { y: '2011', a: 75,  b: 65 },
      { y: '2012', a: 100, b: 90 }
    ],
    xkey: 'y',
    ykeys: ['a', 'b'],
    labels: ['Series A', 'Series B']
  });
  </code></pre>
  
  <p><a href="http://jsbin.com/otaxef/115/embed?javascript,live">Area Chart Example</a> </p>
  
  <pre><code>Morris.Area({
    element: 'area-example',
    data: [
      { y: 'dddd', a: 100, b: 90 },
      { y: '2007', a: 75,  b: 65 },
      { y: '2008', a: 50,  b: 40 },
      { y: '2009', a: 75,  b: 65 },
      { y: '2010', a: 50,  b: 40 },
      { y: '2011', a: 75,  b: 65 },
      { y: '2012', a: 100, b: 90 }
    ],
    xkey: 'y',
    ykeys: ['a', 'b'],
    labels: ['Series A', 'Series B']
  });
  </code></pre>
  
  <p>In Morris Chart library Bar charts accepts strings for Y-AXIS  but Area one doesn't.I can't understand  why ?</p>
  
  <p><a href="http://www.oesmith.co.uk/morris.js/bars.html">THIS IS THE MORRIS LIBRARY</a>  </p>