public function createChart($chart_type)
 {
     $chart = null;
     if ($chart_classname = $this->getChartClassname($chart_type)) {
         $dao = new GraphOnTrackers_ChartDao(CodendiDataAccess::instance());
         $default_title = 'Untitled ' . $chart_type;
         $default_description = '';
         $default_width = call_user_func(array($chart_classname, 'getDefaultWidth'));
         $default_height = call_user_func(array($chart_classname, 'getDefaultHeight'));
         $id = $dao->create($this->id, $chart_type, $default_title, $default_description, $default_width, $default_height);
         $rank = $dao->getRank($id);
         $chart = call_user_func(array($chart_classname, 'create'), $this, $id, $rank, $default_title, $default_description, $default_width, $default_height);
     }
     return $chart;
 }