def average(self, rows: List[Row], column: NumberColumn) -> Number:
        """
        Takes a list of rows and a column and returns the mean of the values under that column in
        those rows.
        """
        cell_values = [row.values[column.name] for row in rows]
        if not cell_values:
            return 0.0  # type: ignore
        return sum(cell_values) / len(cell_values)