PlantUML

PlantUML is an open-source, plain-text domain-specific language for describing diagrams as code. Instead of drawing shapes by hand, you write a compact textual description between @startuml and @enduml markers, and a renderer produces the picture. The language is declarative and line-oriented, so diagrams live in version control alongside source code and read as diffable text.

It covers the main UML diagram types (sequence, use case, class, object, activity, component, deployment, state, and timing) plus a broad set of non-UML notations: entity-relationship and information-engineering diagrams, network diagrams, Gantt charts, mind maps, work-breakdown structures, C4 models, ArchiMate, BPMN, salt GUI wireframes, and direct rendering of JSON and YAML data. Layout of graph-like diagrams (class, component, state, deployment) is delegated to Graphviz, whose DOT engine positions nodes and edges; sequence, activity and timing diagrams use PlantUML's own layout. It is written in Java, licensed GPL, and exports PNG, SVG, LaTeX/TikZ, and ASCII art.

The typical use case is docs-as-code: embedding diagrams in READMEs, wikis (Confluence), and IDEs (VS Code, IntelliJ, Eclipse), and rendering them in CI or via a gateway like Kroki. Strengths are its very wide diagram coverage, mature tooling, and text-based reviewability. Limitations are a Java and Graphviz dependency, layouts that can be hard to fine-tune, and a large but idiosyncratic syntax that varies per diagram type. Comparable text-to-diagram formats include Mermaid, D2, and Structurizr DSL.