Google Maps API v3:首先加载2个位置的地图上的驾驶方向

我有一个地图，它用标记显示了两个位置(并自动在这两个位置之间居中)，您可以在这里看到它:http://goo.gl/rRDCE我正在尝试将它与方向服务集成，就像这个演示一样:https://developers.google.com/maps/documentation/javascript/examples/directions-simple

地图应该首先加载两个位置标记，然后如果用户选择一个起点，它应该显示到两个位置之一的方向。

这是我到目前为止所做的(当然不工作，地图没有加载)…: Phttp://jsfiddle.net/multiformeingegno/fhAJA/5/

var directionsDisplay;
var directionsService = new google.maps.DirectionsService();
var map;
function initialize() {
    directionsDisplay = new google.maps.DirectionsRenderer();
var locations = [
    ['Place 1', 41.897467, 12.470364, 2],
    ['Place 2', 41.896561, 12.467792, 1]
];
    var infowindow = new google.maps.InfoWindow();
    var bounds = new google.maps.LatLngBounds();
    for (i = 0; i < locations.length; i++) {
        marker = new google.maps.Marker({
            position: new google.maps.LatLng(locations[i][1], locations[i][2]),
            map: map
        });
        bounds.extend(marker.position);
        google.maps.event.addListener(marker, 'click', (function (marker, i) {
            return function () {
                infowindow.setContent(locations[i][0]);
                infowindow.open(map, marker);
            }
        })(marker, i));
    }
    map.fitBounds(bounds);
    var listener = google.maps.event.addListener(map, "idle", function () {
        map.setZoom(17);
        google.maps.event.removeListener(listener);
    });
    directionsDisplay.setMap(map);
}
function calcRoute() {
    var start = document.getElementById('start').value;
    var end = document.getElementById('end').value;
    var request = {
        origin: start,
        destination: end,
        travelMode: google.maps.DirectionsTravelMode.DRIVING
    };
    directionsService.route(request, function (response, status) {
        if (status == google.maps.DirectionsStatus.OK) {
            directionsDisplay.setDirections(response);
        }
    });
}
google.maps.event.addDomListener(window, 'load', initialize);

<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false"></script>
<b>Start: </b>
<select id="start" onchange="calcRoute();">
    <option value="chicago, il">Chicago</option>
    <option value="st louis, mo">St Louis</option>
    <option value="joplin, mo">Joplin, MO</option>
    <option value="oklahoma city, ok">Oklahoma City</option>
    <option value="amarillo, tx">Amarillo</option>
    <option value="gallup, nm">Gallup, NM</option>
    <option value="flagstaff, az">Flagstaff, AZ</option>
    <option value="winona, az">Winona</option>
    <option value="kingman, az">Kingman</option>
    <option value="barstow, ca">Barstow</option>
    <option value="san bernardino, ca">San Bernardino</option>
    <option value="los angeles, ca">Los Angeles</option>
</select> <b>End: </b>
<select id="end" onchange="calcRoute();">
    <option value="41.897467, 12.470364">Place 1</option>
    <option value="41.896561, 12.467792">Place 2</option>
</select>
<div id="map" style="position:relative;width:500px;height:300px"></div>

我做错了什么?:)