Toggle opacity of map group item

by amcharts

HTML

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

CSS

body {
    font-family: Verdana;
    font-size: 12px;
    padding: 10px;
}
#chartdiv {
	width	: 100%;
	height	: 500px;
    position: relative;
}
.amChartsLegend path, .amChartsLegend rect {
    cursor: pointer;
}
}

JavaScript

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

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

	dataProvider: {
		map: "worldLow",
		zoomLevel: 3.5,
		zoomLongitude: 10,
		zoomLatitude: 52,
		areas: [{
				title: "Austria",
				id: "AT",
				color: "#67b7dc",
				customData: "1995",
				groupId: "before2004"
			}, {
				title: "Ireland",
				id: "IE",
				color: "#67b7dc",
				customData: "1973",
				groupId: "before2004"
			}, {
				title: "Denmark",
				id: "DK",
				color: "#67b7dc",
				customData: "1973",
				groupId: "before2004"
			}, {
				title: "Finland",
				id: "FI",
				color: "#67b7dc",
				customData: "1995",
				groupId: "before2004"
			}, {
				title: "Sweden",
				id: "SE",
				color: "#67b7dc",
				customData: "1995",
				groupId: "before2004"
			}, {
				title: "Great Britain",
				id: "GB",
				color: "#67b7dc",
				customData: "1973",
				groupId: "before2004"
			}, {
				title: "Italy",
				id: "IT",
				color: "#67b7dc",
				customData: "1957",
				groupId: "before2004"
			}, {
				title: "France",
				id: "FR",
				color: "#67b7dc",
				customData: "1957",
				groupId: "before2004"
			}, {
				title: "Spain",
				id: "ES",
				color: "#67b7dc",
				customData: "1986",
				groupId: "before2004"
			}, {
				title: "Greece",
				id: "GR",
				color: "#67b7dc",
				customData: "1981",
				groupId: "before2004"
			}, {
				title: "Germany",
				id: "DE",
				color: "#67b7dc",
				customData: "1957",
				groupId: "before2004"
			}, {
				title: "Belgium",
				id: "BE",
				color: "#67b7dc",
				customData: "1957",
				groupId: "before2004"
			}, {
				title: "Luxembourg",
				id: "LU",
				color: "#67b7dc",
				customData: "1957",
				groupId: "before2004"
			}, {
				title: "Netherlands",
				id: "NL",
				color: "#67b7dc",
				customData: "1957",
				groupId: "before2004"
			}, {
				title: "Portugal",
				id: "PT",
				color: "#67b7dc",
				customData: "1986",
				groupId:...