Hello PlantUML team,
Like @coxley on QA-11489, for Gantt, here is for JSON, (we did not check for PNG [Please to confirm] but only for SVG), I just copy with change the diagram type [thank @coxley for the canvas] 
:
"I understand this syntax is experimental. But in case the team isn't aware, [JSON] diagrams don't generate metadata in PNG [to confirm or not] or SVG [we confirm, no metadata observed on SVG] renders.
Low priority.
Thanks for everything you do. :)"
Thank for your support, and thank for this new feature, 
Regards,
Th.