我想叠加地图，
init: function (self) {
layerChina400 = new SuperMap.Layer.TiledDynamicRESTLayer("China400", url, {transparent: true,cacheEnabled: true}, {maxResolution: "auto"});
layerChina = new SuperMap.Layer.TiledDynamicRESTLayer("China", url2, {transparent: true, cacheEnabled: true}, {maxResolution:"auto"});
layerChina400.events.on({"layerInitialized": self.addLayerIndex});
},
addLayerIndex: function () {
layerIndex = new SuperMap.Layer.TiledDynamicRESTLayer("JS", url3, {transparent: true, cacheEnabled: true});
layerIndex.events.on({"layerInitialized": self.initEnd});
layerIndex.setOpacity(0.5);
},
initEnd: function () {
map.addLayers([layerChina400,layerIndex]);
map.setCenter(new SuperMap.LonLat(0, 0), 0);
layerIndex.isBaseLayer = false;
console.log(map.layers);
},
怎么样才能把这个单选钮变成多选钮啊，有两张底图了，单选钮地图不能叠加显示啊
