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Hello D.,
This functionality is now available from Nov. 2020, and version ~V1.2020.20 (20 Nov, 2020):
[PlantUML team] have added a timeline footer: this is a test to see if people like it or not.
See: https://forum.plantuml.net/12289/gantt-printscale-weekly-coloring-current-week?show=12341#c12341
[See on PlantUML server]
And you can deactivate it with:
hide footbox
If that can help,Regards,Th.