Mermaid

Mermaid turns plain text into diagrams: flowcharts, sequence diagrams, ER diagrams, Gantt charts, plus newer types like architecture diagrams, kanban boards and Wardley maps. It's MIT, and GitHub, GitLab, Obsidian and Notion render it natively, so a code block in a README becomes a picture with no image file to keep in sync.

Cost to run: zero. Drop the library into a page, or do nothing when your platform already renders it. Version 12 (September 2026) made ELK the default layout engine, which re-lays out existing flowcharts; set layout: dagre to keep the old layout.

The paid side is Mermaid Chart at mermaid.ai, a hosted editor with AI credits, billed annually only. Plus is $10 per user per month, Premium $20, and SSO now lives in custom-priced Enterprise. Solo and small teams: free library, skip the editor. Larger teams pay only when non-technical people want a visual editor or SSO.

The catch: you steer the layout, you don't place nodes, so big diagrams need restructuring when the engine guesses wrong. For polished presentation work, Excalidraw wins. Rendering diagrams from untrusted users? Stay current: 2026 brought nine medium and low severity advisories.