Mermaid

Mermaid turns plain text into diagrams: flowcharts, sequence diagrams, Gantt charts, ERDs, Git graphs. You type a few lines of structured text and get a diagram. It lives in your markdown files, your docs, your README.

MIT license. GitHub renders Mermaid natively in markdown files. So does Notion, Obsidian, GitLab, and dozens of other tools. Write a diagram in a code block, push it, and it shows up rendered. No image files to maintain, no external tool to open.

Everything is free. The library is MIT-licensed with no paid tier. Mermaid Chart (the commercial product from the same team) offers a visual editor and collaboration features starting at $8/user/mo, but the rendering engine and syntax are fully open source.

The catch: complex diagrams get messy fast. Once you hit 30+ nodes, the auto-layout makes questionable decisions and you can't manually position things. For polished presentation diagrams, you'll still reach for Figma or draw.io. Mermaid is best for documentation-grade diagrams that live alongside code, not boardroom slides.