function initIndexMap(arr, map, keyArr, keyGetter) {
	        for (var i = 0; i < arr.length; i++) {
	            // Add prefix to avoid conflict with Object.prototype.
	            var key = '_ec_' + keyGetter(arr[i], i);
	            var existence = map[key];
	            if (existence == null) {
	                keyArr.push(key);
	                map[key] = i;
	            }
	            else {
	                if (!existence.length) {
	                    map[key] = existence = [existence];
	                }
	                existence.push(i);
	            }
	        }
	    }
Get width of band
@return {number}