The Excel worksheet reference to the Y values for this chart (not
        including the column label).

def y_values_ref(self, series):
        """
        The Excel worksheet reference to the Y values for this chart (not
        including the column label).
        """
        top_row = self.series_table_row_offset(series) + 2
        bottom_row = top_row + len(series) - 1
        return "Sheet1!$B$%d:$B$%d" % (top_row, bottom_row)