public function load_choices() {
        if (is_array($this->choices)) {
            return true;
        }
        $this->choices = array();

        global $CFG;
        require_once($CFG->libdir.'/gradelib.php');

        foreach (core_component::get_plugin_list('gradereport') as $plugin => $plugindir) {
            if (file_exists($plugindir.'/lib.php')) {
                require_once($plugindir.'/lib.php');
                $functionname = 'grade_report_'.$plugin.'_profilereport';
                if (function_exists($functionname)) {
                    $this->choices[$plugin] = get_string('pluginname', 'gradereport_'.$plugin);
                }
            }
        }
        return true;
    }