title: c3.js multi-line charts with missing data/different x values
body: <p>I have problem with c3.js with mutli-line chart and different x values in every line. 
  For example:</p>
  
  <pre><code> data1 = [
    {
      "date": 1470520800000,
      "admin": 10
    },
    {
      "date": 1470607200000,
      "admin": 10.06
    },
    {
      "date": 1470693600000,
      "admin": 10.03
    },
    {
      "date": 1470780000000,
      "admin": 10
    },
    {
      "date": 1470866400000,
      "admin": 10.05
    },
    {
      "date": 1470952800000,
      "admin": 10
    }
  ];
  
  data2 = [
    {
      "date": 1470607200000,
      "user": 94.66
    },
    {
      "date": 1470693600000,
      "user": 316.58
    },
    {
      "date": 1470780000000,
      "user": 10
    },
    {
      "date": 1470866400000,
      "user": 1.5
    },
    {
      "date": 1470952800000,
      "user": 12
    }
  ];
  
      var chart = c3.generate({
          data: {
              json: data1,
              keys: {
                  x: 'date',
                  value: ["admin"]
              }
          },
          axis: {
              x: {
                  type: 'timeseries',
                  tick: {
                      format: function (x) { return x.toLocaleDateString(); }
                  }
              }
          },
          padding: {
              right: 50
          }
      });
  
      chart.load({
                  json: data2,
                  keys: {
                      x: "date",
                      value: ['user']
                  }
              });
  </code></pre>
  
  <p>Data does not display well when i fucus on them with my mouse.
  I post my code here <a href="https://jsfiddle.net/gahm5wgb/" rel="nofollow">https://jsfiddle.net/gahm5wgb/</a>.</p>