private function processHistoricalResponse(Response $response)
 {
     if ($response->isOK()) {
         $data = json_decode($response->getContent(true), true);
         $values = array_map(array($this, 'float'), $data['val']);
         array_pop($values);
         // remove the last null value...
         return new History(new DateTime($data['tm']), $values, $data['un'], (int) $data['dt']);
     }
 }