XML error in generated xlsx file after using page break in worksheets with charts
*Created originally on Bitbucket by [liuyuqian](https://bitbucket.org/%7B987af662-7ddf-415a-b86c-03da3f6cb95f%7D/)*
Use line chart demo code, then add a page break.
Generated xlsx file can be read with WPS or LibreOffice,and page break work well,but raise a XML error after Excel 2010 & 2016 fixed the file.
Environment:Python 3.5 openpyxl 2.4.1 Excel 2010 & 2016
*Attachments:* [linecharts.py](/uploads/d23fdedaab0dda78e29fe8a61184e207/linecharts.py)[line.xlsx](/uploads/df38545745381a8bada4801559649d30/line.xlsx)
issue