Show average line on Stock Chart based on current selection

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

CSS

body {
    font-size: 12px;
    font-family: Verdana;
    padding: 10px;
}
#chartdiv {
	width	: 100%;
	height	: 500px;
}

JavaScript

var chartData1 = generateChartData();
var chartData2 = generateChartData();
function generateChartData() {
	var chartData = [];
	var firstDate = new Date(2012, 0, 1);
	firstDate.setDate(firstDate.getDate() - 1000);
	firstDate.setHours(0, 0, 0, 0);

	for (var i = 0; i < 1000; i++) {
		var newDate = new Date(firstDate);
		newDate.setHours(0, i, 0, 0);

		var a = Math.round(Math.random() * (40 + i)) + 100 + i;
		var b = Math.round(Math.random() * 100000000);

		chartData.push({
			date: newDate,
			value: a,
			volume: b
		});
	}
	return chartData;
}


var chart = AmCharts.makeChart("chartdiv", {

		type: "stock",
        "theme": "none",
        pathToImages: "http://www.amcharts.com/lib/3/images/",
    glueToTheEnd: true,

		categoryAxesSettings: {
			minPeriod: "mm"
		},

		dataSets: [{
			color: "#b0de09",
			fieldMappings: [{
				fromField: "value",
				toField: "value"
			}, {
				fromField: "volume",
				toField: "volume"
			}],

			dataProvider: chartData1,
			categoryField: "date"
		},{
			color: "#b0de09",
			fieldMappings: [{
				fromField: "value",
				toField: "value"
			}, {
				fromField: "volume",
				toField: "volume"
			}],

			dataProvider: chartData2,
			categoryField: "date"
		}],


		panels: [{
				showCategoryAxis: false,
				title: "Value",
				percentHeight: 70,

				stockGraphs: [{
					id: "g1",
					valueField: "value",
					type: "smoothedLine",
					lineThickness: 2,
					bullet: "round"
				}],


				stockLegend: {
					valueTextRegular: " ",
					markerType: "none"
				},
            
            valueAxes: [{
                guides: [{
                    value: 1100,
                    label: "AVERAGE",
                    position: "right",
                    dashLength: 5
                }]
            }]
			},

			{
				title: "Volume",
				percentHeight: 30,
				stockGraphs: [{
					valueField: "volume",
					type: "column",
					cornerRadiusTop: 2,
					fillAlphas: 1
				}],

				stockLegend: {
					valueTextRegular: "...