mermaid.js is now build in to Notion.
Use /mermaid command in Notion to write inline UML diagrams.
Compose Notion PlantUML diagrams with the NoMunchies app.
Write PlantUML in the companion app, press build, and upload the diagram image to Notion.
The user can define the page and the block where the diagram will be uploaded.
Uploading is fast, about 3s.