An open-source, Public Domain diagramming software.
Draw.io is open source, available as a Progressive Web App and pretty cool. Like Excalidraw, it can make graphics for free using just a browser - but DrawIO is more like Visio.
There are free integrations for DrawIO in both Notion and Obsidian.
☝️ added with Command Palette: New Diagram, can be updated with right-click -> edit diagram.
Notion actually isn’t natively supported. Just embeds work well. When exporting as URL. Also I guess DrawIO offers a Chrome Extension for working with Notion.