Returns the query object for this visualization

def query_obj(self):
        """Returns the query object for this visualization"""
        d = super().query_obj()
        d['row_limit'] = self.form_data.get(
            'row_limit', int(config.get('VIZ_ROW_LIMIT')))
        numeric_columns = self.form_data.get('all_columns_x')
        if numeric_columns is None:
            raise Exception(_('Must have at least one numeric column specified'))
        self.columns = numeric_columns
        d['columns'] = numeric_columns + self.groupby
        # override groupby entry to avoid aggregation
        d['groupby'] = []
        return d