protected function getUsings()
    {
        $usings = array();

        for ($i=0; $i<count($this->values); $i++) {
            $using = '"-" using 1:2 with '.$this->mode;
            if (isset($this->titles[$i])) {
                $using .= ' title "'.$this->titles[$i].'"';
            }
            $usings[] = $using;
        }

        return implode(', ', $usings);
    }