public function trackEvent($category, $action, $label = null, $value = null)
	{
		$item = "ga('send', 'event', '{$category}', '{$action}'" .
			($label !== null ? ", '{$label}'" : '') .
			($value !== null && is_numeric($value) ? ", {$value}" : '') .
			");";
		$this->addItem($item);
	}