public static function getUnit($column, $idSite)
    {
        $nameToUnit = array(
            '_rate'   => '%',
            'revenue' => Site::getCurrencySymbolFor($idSite),
            '_time_'  => 's'
        );

        $unit = null;

        /**
         * Use this event to define units for custom metrics used in evolution graphs and row evolution only.
         *
         * @param string $unit should hold the unit (e.g. %, €, s or empty string)
         * @param string $column name of the column to determine
         * @param string $idSite id of the current site
         */
        Piwik::postEvent('Metrics.getEvolutionUnit', [&$unit, $column, $idSite]);

        if (!empty($unit)) {
            return $unit;
        }

        foreach ($nameToUnit as $pattern => $type) {
            if (strpos($column, $pattern) !== false) {
                return $type;
            }
        }

        return '';
    }