Mermaid

Mermaid lets you write diagrams in Markdown-like text and render them as SVGs. Flowcharts, sequence diagrams, ER diagrams, Gantt charts — all from text that lives in your Git repo. GitHub, GitLab, Notion, and Obsidian all render Mermaid natively now.

PlantUML is the older alternative with more diagram types but uglier output and Java dependency. draw.io/diagrams.net is the visual editor for people who want to drag boxes. Excalidraw is for hand-drawn-style diagrams. Lucidchart is the enterprise commercial option.

If you're writing technical docs or ADRs and need diagrams that version-control alongside code, Mermaid is the answer. No external tools, no image files to manage, no binary formats. The syntax is intuitive enough that non-engineers can contribute.

The catch: complex diagrams get unwieldy fast. The text syntax doesn't scale to 50-node flowcharts — you'll want a visual editor at that point. Rendering can be inconsistent across platforms, and styling options are limited compared to dedicated diagramming tools.