c3 - JSON data with timeseries and date filter

Display only the first of the month

HTML

<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/c3/0.4.6/c3.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/c3/0.4.6/c3.js"></script>
<div id="chart"></div>

JavaScript

var chart = c3.generate({
                        bindto: '#chart',
                        data: {
                            json:  [ 
                                    {"_id":1417600089156,"a":6,"b":10,"c":6,"d":20},         
                                    {"_id":1407600089156,"a":4,"b":0,"c":4,"d":9}, 
                                    {"_id":1397600089156,"a":30,"b":25,"c":20,"d":15} 
                                    ],
                            keys: {
                               x:'_id',                                
                                value: ['a', 'b','c','d'],
                            }
                        },
                        axis : {
                            x : {
                                type : 'timeseries',
                                culling: false,
                                tick: {
                                    format: function(time){
                                        console.log('time '+time);
                                        var dat = new Date(time);
                                        var _months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sept", "Oct", "Nov", "Dec"];
                                        var month = dat.getMonth();
                                        
                                        
                                        return _months[month];
                                    }
                                }                                 
                            }
                        }
                    });