As far as I understand bootchart2 is bundled with a python script called pybootchartgui to visualize the bootchart logs. There is a short description on the Gentoo Wiki page for that tool. The user expects that tool somewhere.
But obviously the script is totally missing with current app-benchmarks/bootchart2-0.14.8-r1 package leaving the user in the dust...
The other option would have been to completely drop the package from the repository. We can still do that. Alternatively, feel free to improve the wiki.
*** Bug 816288 has been marked as a duplicate of this bug. ***
I believe this August's update to the project makes it viable to include pybootchartgui again (since it was ported to Python 3.8+ and the pygtk dependency was made optional).
Resolution of this bug depends on it getting a new maintainer. Feel free to
a) provide links to these changes you mentioned so anyone interested can get going
b) provide a patch or PR to the ebuild yourself