The Excel worksheet reference to the range containing the bubble
        sizes for *series* (not including the column heading cell).

def bubble_sizes_ref(self, series):
        """
        The Excel worksheet reference to the range containing the bubble
        sizes for *series* (not including the column heading cell).
        """
        top_row = self.series_table_row_offset(series) + 2
        bottom_row = top_row + len(series) - 1
        return "Sheet1!$C$%d:$C$%d" % (top_row, bottom_row)