def _write_chartsheet_rels_files(self):
        # Write the chartsheet .rels files for links to drawing files.
        index = 0
        for worksheet in self.workbook.worksheets():

            if not worksheet.is_chartsheet:
                continue

            index += 1

            external_links = worksheet.external_drawing_links

            if not external_links:
                continue

            # Create the chartsheet .rels xlsx_dir.
            rels = Relationships()

            for link_data in external_links:
                rels._add_worksheet_relationship(*link_data)

            # Create .rels file such as /xl/chartsheets/_rels/sheet1.xml.rels.
            rels._set_xml_writer(self._filename('xl/chartsheets/_rels/sheet'
                                                + str(index) + '.xml.rels'))
            rels._assemble_xml_file()

    def _write_drawing_rels_files(self):
        # Write the drawing .rels files for worksheets with charts or drawings.

        xlsx_dir = self.package_dir

        index = 0
        for worksheet in self.workbook.worksheets():
            if not worksheet.drawing_links:
                continue
            index += 1

            # Create the drawing .rels xlsx_dir.
            self._mkdir(xlsx_dir + '/xl')
            self._mkdir(xlsx_dir + '/xl/drawings')
            self._mkdir(xlsx_dir + '/xl/drawings/_rels')

            rels = Relationships()

            for drawing_data in worksheet.drawing_links:
                rels._add_document_relationship(*drawing_data)

            # Create .rels file such as /xl/drawings/_rels/sheet1.xml.rels.
            rels._set_xml_writer(xlsx_dir + '/xl/drawings/_rels/drawing'
                                 + str(index) + '.xml.rels')
            rels._assemble_xml_file()

    def _write_worksheet_rels_files(self):
        # Write data such as hyperlinks or drawings.
        index = 0
        for worksheet in self.workbook.worksheets():

            if worksheet.is_chartsheet:
                continue

            index += 1

            external_links = (worksheet.external_hyper_links +
                              worksheet.external_drawing_links +
                              worksheet.external_vml_links +
                              worksheet.external_table_links +
                              worksheet.external_comment_links)

            if not external_links:
                continue

            # Create the worksheet .rels dirs.
            rels = Relationships()

            for link_data in external_links:
                rels._add_worksheet_relationship(*link_data)

            # Create .rels file such as /xl/worksheets/_rels/sheet1.xml.rels.
            rels._set_xml_writer(self._filename('xl/worksheets/_rels/sheet'
                                                + str(index) + '.xml.rels'))
            rels._assemble_xml_file()

    def _write_vml_drawing_rels_file(self, worksheet, index):
        # Write the vmlDdrawing .rels files for worksheets with images in
        # headers or footers.

        # Create the drawing .rels dir.
        rels = Relationships()

        for drawing_data in worksheet.vml_drawing_links:
            rels._add_document_relationship(*drawing_data)

        # Create .rels file such as /xl/drawings/_rels/vmlDrawing1.vml.rels.
        rels._set_xml_writer(self._filename("xl/drawings/_rels/vmlDrawing" + str(index) + ".vml.rels"))
        rels._assemble_xml_file()

    def _write_drawing_rels_files(self):
        # Write the drawing .rels files for worksheets with charts or drawings.
        index = 0
        for worksheet in self.workbook.worksheets():
            if not worksheet.drawing_links:
                continue
            index += 1

            # Create the drawing .rels xlsx_dir.
            rels = Relationships()

            for drawing_data in worksheet.drawing_links:
                rels._add_document_relationship(*drawing_data)

            # Create .rels file such as /xl/drawings/_rels/sheet1.xml.rels.
            rels._set_xml_writer(self._filename("xl/drawings/_rels/drawing" + str(index) + ".xml.rels"))
            rels._assemble_xml_file()

    def _write_workbook_rels_file(self):
        # Write the _rels/.rels xml file.
        rels = Relationships()

        worksheet_index = 1
        chartsheet_index = 1

        for worksheet in self.workbook.worksheets():
            if worksheet.is_chartsheet:
                rels._add_document_relationship('/chartsheet',
                                                'chartsheets/sheet'
                                                + str(chartsheet_index)
                                                + '.xml')
                chartsheet_index += 1
            else:
                rels._add_document_relationship('/worksheet',
                                                'worksheets/sheet'
                                                + str(worksheet_index)
                                                + '.xml')
                worksheet_index += 1

        rels._add_document_relationship('/theme', 'theme/theme1.xml')
        rels._add_document_relationship('/styles', 'styles.xml')

        # Add the sharedString rel if there is string data in the workbook.
        if self.workbook.str_table.count:
            rels._add_document_relationship('/sharedStrings',
                                            'sharedStrings.xml')

        # Add vbaProject if present.
        if self.workbook.vba_project:
            rels._add_ms_package_relationship('/vbaProject', 'vbaProject.bin')

        rels._set_xml_writer(self._filename('xl/_rels/workbook.xml.rels'))
        rels._assemble_xml_file()

    def _write_root_rels_file(self):
        # Write the _rels/.rels xml file.
        rels = Relationships()

        rels._add_document_relationship('/officeDocument', 'xl/workbook.xml')
        rels._add_package_relationship('/metadata/core-properties',
                                       'docProps/core.xml')
        rels._add_document_relationship('/extended-properties',
                                        'docProps/app.xml')

        rels._set_xml_writer(self._filename('_rels/.rels'))
        rels._assemble_xml_file()

    def _write_workbook_rels_file(self):
        # Write the _rels/.rels xml file.
        rels = Relationships()

        worksheet_index = 1
        chartsheet_index = 1

        for worksheet in self.workbook.worksheets():
            if worksheet.is_chartsheet:
                rels._add_document_relationship("/chartsheet", "chartsheets/sheet" + str(chartsheet_index) + ".xml")
                chartsheet_index += 1
            else:
                rels._add_document_relationship("/worksheet", "worksheets/sheet" + str(worksheet_index) + ".xml")
                worksheet_index += 1

        rels._add_document_relationship("/theme", "theme/theme1.xml")
        rels._add_document_relationship("/styles", "styles.xml")

        # Add the sharedString rel if there is string data in the workbook.
        if self.workbook.str_table.count:
            rels._add_document_relationship("/sharedStrings", "sharedStrings.xml")

        # Add vbaProject if present.
        if self.workbook.vba_project:
            rels._add_ms_package_relationship("/vbaProject", "vbaProject.bin")

        rels._set_xml_writer(self._filename("xl/_rels/workbook.xml.rels"))
        rels._assemble_xml_file()