Simulating date-based axis on XY chart using labelFunction and timestamps as values

HTML

<script type="text/javascript" src="http://www.amcharts.com/lib/3/amcharts.js"></script>
<script type="text/javascript" src="http://www.amcharts.com/lib/3/xy.js"></script>
<script type="text/javascript" src="http://www.amcharts.com/lib/3/themes/none.js"></script>
<div id="chartdiv"></div>

CSS

#chartdiv {
	width		: 100%;
	height		: 500px;
	font-size	: 11px;
}

JavaScript

var chart = AmCharts.makeChart("chartdiv", {
    "type": "xy",
    "theme": "none",
    "pathToImages": "http://www.amcharts.com/lib/3/images/",
    "dataProvider": [{
        "ay": 0.5,
        "by": 2.2,
        "date": 1406840400000
    }, {
        "ay": 1.3,
        "by": 4.9,
        "date": 1406926800000
    }, {
        "ay": 2.3,
        "by": 5.1,
        "date": 1407013200000
    }, {
        "ay": 2.8,
        "by": 5.3,
        "date": 1407099600000
    }, {
        "ay": 3.5,
        "by": 6.1,
        "date": 1407186000000
    }, {
        "ay": 5.1,
        "by": 8.3,
        "date": 1407272400000
    }, {
        "ay": 6.7,
        "by": 10.5,
        "date": 1407358800000
    }, {
        "ay": 8,
        "by": 12.3,
        "date": 1407445200000
    }, {
        "ay": 8.9,
        "by": 14.5,
        "date": 1407531600000
    }, {
        "ay": 9.7,
        "by": 15,
        "date": 1407704400000
    }, {
        "ay": 10.4,
        "by": 18.8,
        "date": 1407790800000
    }, {
        "ay": 11.7,
        "by": 19,
        "date": 1407877200000
    }],
    "valueAxes": [{
        "position":"bottom",
        "axisAlpha": 0,
        "dashLength": 1,
        "title": "X Axis (simulated date)",
        "minimum": 1406840400000,
        "maximum": 1407877200000,
        "labelFunction": function (value) {
            var date = new Date(value);
            return AmCharts.formatDate(date, "MMM DD");
        }
    }, {
        "axisAlpha": 0,
        "dashLength": 1,
        "position": "left",
        "title": "Y Axis"
    }],
    "startDuration": 1,
    "graphs": [{
        "balloonText": "x:[[x]] y:[[y]]",
        "bullet": "triangleUp",
        "lineAlpha": 0,
        "xField": "date",
        "yField": "ay",
        "lineColor": "#FF6600",
    "fillAlphas": 0
    }, {
        "balloonText": "x:[[x]] y:[[y]]",
        "bullet": "triangleDown",
        "lineAlpha": 0,
        "xField": "date",
        "yField": "by",
        "lineColor":...