Long Story Short: PR accepted by PHPOffice/PhpSpreadsheet maintainers. So it will be included in next release.
We use both JpGraph and PhpSpreadsheet intensively in our projects. But they were not intersected at all since we do not add charts to our spreadsheets.
PHPOffice/PhpSpreadsheet used old abandoned jpgraph/jpgraph package to render charts in spreadsheets so there were no possibility to run it under PHP 8.1 without additional dances, drumming and code-patching.
One of users asked us if our version of JpGraph compatible with PhpSpreadsheet. Answer was yes... but. Oh, there is always but. Investigation led us deep into PhpSpreadsheet's source code, solution was found and confirmed working. So we created pull-request and now it is accepted. The goal was not to break existing compatibility with old jpgraph/jpgraph. So now both abandoned jpgraph/jpgraph and actual mitoteam/jpgraph versions are supported by PhpSpreadsheet.
Have a nice coding!