c3 - JSON data with timeseries and date filter

Display only the first of the month

HTML

<script src="https://rawgit.com/masayuki0812/c3/master/c3.js"></script>
<link rel="stylesheet" href="https://rawgit.com/masayuki0812/c3/master/c3.css">
<script src="http://gabelerner.github.io/canvg/canvg.js"></script>
<script src="https://gabelerner.github.io/canvg/StackBlur.js"></script>
<script src="https://gabelerner.github.io/canvg/rgbcolor.js"></script>
<div id="chart"></div>
<canvas id='canvas' width="1000px" height="600px"/>

JavaScript

var chart = c3.generate({
    data: {
        json: [{
            date: '2014-01-01',
            upload: 200,
            download: 200,
            total: 400
        }, {
            date: '2014-01-02',
            upload: 100,
            download: 300,
            total: 400
        }, {
            date: '2014-02-01',
            upload: 300,
            download: 200,
            total: 500
        }, {
            date: '2014-02-02',
            upload: 400,
            download: 100,
            total: 500
        }],
        keys: {
            x: 'date',
            value: ['upload', 'download']
        }
    },
    axis: {
        x: {
            type: 'timeseries',
            tick: {
                format: function (x) {
                    if (x.getDate() === 1) {
                        return x.toLocaleDateString();
                    }
                }
            }
        }
    }
});

canvg('canvas', document.getElementById('chart').innerHTML);

// the canvas calls to output a png
var canvas = document.getElementById("canvas");
var img = canvas.toDataURL("image/png");