Hello,
phpdocx allows changing data chart title, legends and values using replaceChartData (https://www.phpdocx.com/api-documentation/docxutilities/replace-chart-data-Word-document).
XLSXUtilities allows changing placeholders in XLSX files, but not in embedded XLSX files, as the API pages detail (https://www.phpdocx.com/api-documentation/xlsxutilities/search-keyword-Excel-document-and-replace-it-with-PHP).
We move the request to the dev team to consider adding support. Using the current version it may require a manual workflow:
1. Open the DOCX and extract the embedded XLSX and change the placeholders.
2. Add the updated XLSX to the DOCX file again.
3. Refresh some internal rels if needed.
If you open a ticket (https://www.phpdocx.com/support), the dev team can generate a script to accomplish this kind of special workflows that aren't supported yet.
Regards.