title: C3 - Timeseries chart with JSON and categories
body: <p>I am using C3 library for the first time and I think it's a good alternative to D3 for designing simple and reusable charts with no pain.</p>
  
  <p>However, I have some issues in designing a timeseries chart.
  Here is an example of the JSON file I will use to generate my chart:</p>
  
  <pre><code>data: {
    json: [
     {
        "city": "Paris",
        "date": "2016-09-01",
        "event": 234
     },
     {
        "city": "Paris",
        "date": "2016-09-02",
        "event": 891
     },
     {
        "city": "Paris",
        "date": "2016-09-03",
        "event": 877
     },
     {
        "city": "Berlin",
        "date": "2016-09-01",
        "event": 190
     },
     {
        "city": "Berlin",
        "date": "2016-09-02",
        "event": 234
     },
     {
        "city": "Berlin",
        "date": "2016-09-03",
        "event": 231
     },
     {
        "city": "London",
        "date": "2016-09-01",
        "event": 23
     },
     {
        "city": "London",
        "date": "2016-09-02",
        "event": 12
     },
     {
        "city": "London",
        "date": "2016-09-03",
        "event": 89
     },
   ],
  </code></pre>
  
  <p>The problem is that I can not set both my axis x: as a timeseries type and the key "city" as a category type.</p>
  
  <p>For now I have:</p>
  
  <pre><code>keys: {
     x: 'period',
     value: ['event'],
  },
  axis: {
   x: {
     type: 'timeseries',
     tick: {
       format: '%Y-%m-%d'
     }
   }
  },
  type: 'spline'
  </code></pre>
  
  <p>And the corresponding Plunker: <a href="http://plnkr.co/edit/T1aLWQpaFwdu2zsWCa3d" rel="nofollow noreferrer">http://plnkr.co/edit/T1aLWQpaFwdu2zsWCa3d</a></p>
  
  <p>I would like to have 3 splines, corresponding to the 3 cities that are retrieved from the JSON file.</p>
  
  <p>Can you help me achieve this ?</p>
  
  <p>Thank you very much :)</p>