Write chart data contents to *worksheet* in the bubble chart layout.
        Write the data for each series to a separate three-column table with
        X values in column A, Y values in column B, and bubble sizes in
        column C. Place the series label in the first (heading) cell of the
        values column.

def _populate_worksheet(self, workbook, worksheet):
        """
        Write chart data contents to *worksheet* in the bubble chart layout.
        Write the data for each series to a separate three-column table with
        X values in column A, Y values in column B, and bubble sizes in
        column C. Place the series label in the first (heading) cell of the
        values column.
        """
        chart_num_format = workbook.add_format(
            {'num_format': self._chart_data.number_format}
        )
        for series in self._chart_data:
            series_num_format = (
                workbook.add_format({'num_format': series.number_format})
            )
            offset = self.series_table_row_offset(series)
            # write X values
            worksheet.write_column(
                offset+1, 0, series.x_values, chart_num_format
            )
            # write Y values
            worksheet.write(offset, 1, series.name)
            worksheet.write_column(
                offset+1, 1, series.y_values, series_num_format
            )
            # write bubble sizes
            worksheet.write(offset, 2, 'Size')
            worksheet.write_column(
                offset+1, 2, series.bubble_sizes, chart_num_format
            )