-if ($group_by_options_size > 2) $form->addInput(array('type'=>'combobox','name'=>'group_by2','data'=>$group_by_options));
-if ($group_by_options_size > 3) $form->addInput(array('type'=>'combobox','name'=>'group_by3','data'=>$group_by_options));
+if ($group_by_options_size > 2) $form->addInput(array('type'=>'combobox','onchange'=>'handleCheckboxes();','name'=>'group_by2','data'=>$group_by_options));
+if ($group_by_options_size > 3) $form->addInput(array('type'=>'combobox','onchange'=>'handleCheckboxes();','name'=>'group_by3','data'=>$group_by_options));