title: Using string for xkey in morrise charts
body: <p>I am trying to use morrise charts to build a line chart that will display vehicles' numbers according to days among the week.</p>
  
  <p>The problem is when I use <strong>String</strong> in the <strong>xKey</strong> the results appears like this:
  <img src="https://i.stack.imgur.com/9hHm0.jpg" alt="enter image description here"></p>
  
  <p>but if I replaced them with numbers, it works fine.</p>
  
  <hr>
  
  <p>Here is my code.</p>
  
  <pre><code>&lt;div class="col-xs-6"&gt;
    &lt;label&gt;Transports Traffic&lt;/label&gt;
    &lt;div id="traffic_chart"&gt;
      &lt;script&gt;
        new Morris.Area({
          element: 'traffic_chart',
            data: [
              {y: 'Sat', a: 100, b: 90, c:22},
              {y: 'Sun', a: 75, b: 65, c:22},
              {y: 'Mon', a: 50, b: 40, c:22},
              {y: 'Tue', a: 75, b: 65, c:22},
              {y: 'Wed', a: 50, b: 40, c:22},
              {y: 'Thi', a: 75, b: 65, c:22},
              {y: 'Fri', a: 100, b: 90, c:22}
            ],
            xkey: 'y',
            ykeys: ['a', 'b', 'c'],
            labels: ['Cars', 'Bikes', 'Trucks']
         });
       &lt;/script&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>