Hello,
The behaviour of phpdocx is the same than the default settings of MS Word. If you generate a DOCX with a chart, then you open the DOCX with MS Word and edit the data doing right click on the chart, if you change a value adding ',0' MS Word removes it automatically.
Changing the format code needs a new attribute that is not supported by the current version of phpdocx (formatCode attribute). We move the request to the dev team to add this feature to a next version of phpdocx.
Regards.