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

def x_values_ref(self, series):
        """
        The Excel worksheet reference to the X 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!$A$%d:$A$%d" % (top_row, bottom_row)