fires when area selection is started
| start | object | the id object of the left top cell, contains two properties: the row id and the column id |
| end | object | the id object of the right cell, contains two properties: the row id and the column id |
| ev | Event object | a native event object |
$$('dtable').attachEvent("onAreaDrag", function(start, end, ev){
// some code here
});