Best Open-Source Diagram Software 2026: 6 Options Compared

Open-source diagramming splits into drag-and-drop editors and diagram-as-code tools, and the right choice depends on how often the diagram changes. Draw.io is the drag-and-drop standard: Apache-2.0, no sign-up, with files saved on your own device or cloud. Excalidraw is the quick, hand-drawn version. Mermaid, PlantUML, D2 and Graphviz let you write text and get a diagram, which fits documentation that lives in Git because the diff shows exactly what changed. Mermaid renders inside many Markdown viewers, PlantUML is strongest for UML sequence and class diagrams and lets you choose from several licenses, D2 is a newer language with a CLI and an online playground, and Graphviz lays out graphs automatically using the DOT language. Which to pick comes down to who edits. If non-technical colleagues must change the diagram, use Draw.io. If engineers maintain it next to code, use a text tool. The main trade-off is layout control: text tools place nodes for you, which is fast and consistent but can produce results you cannot nudge by hand. Graphviz is also an engine behind other tools, including PlantUML's default layout, so you may install it anyway.

draw.io (diagrams.net)

Apache-2.0 editor, free with no sign-up, running as a web app and on Windows, macOS and Linux. Full manual control, but diagrams are not diff-friendly unless you use its XML exports.

Price: Free (Free with no sign-up; open files from local storage or your own cloud account) (draw.io (diagrams.net) pricing, checked 2026-10-22).

Platforms and license: Web, Windows, macOS, Linux. Apache-2.0 (open source) (license source).

Project activity: 9k GitHub stars · latest release v32.6.1 (Oct 23, 2026) · last commit Oct 23, 2026

Best for: Drag-and-drop diagrams for any audience.

Compare: draw.io (diagrams.net) alternatives

Excalidraw

MIT-licensed web whiteboard with a hand-drawn look and real-time collaboration. Quick, but there is no automatic layout or formal notation.

Price: Free (Free web app with real-time collaboration and end-to-end encryption); paid from $6 per user/month, Excalidraw+ (Excalidraw pricing, checked 2026-10-22).

Platforms and license: Web. MIT (open source) (license source).

Project activity: 134.7k GitHub stars · latest release v0.18.1 (Apr 21, 2026) · last commit Oct 27, 2026

Best for: Sketchy diagrams for discussion.

Worth knowing: Excalidraw+ has a 14-day trial; annual billing saves 14%. Hand-drawn look, not a formal-shape library.

Mermaid

MIT library plus the free Mermaid Live Editor. Text becomes flowcharts and sequence diagrams inside Markdown, but you have limited control over placement.

Price: Free (Open-source library plus free online Mermaid Live Editor) (Mermaid pricing, checked 2026-10-22).

Platforms and license: Web, JavaScript library. MIT (open source) (license source).

Project activity: 90.9k GitHub stars · latest release @mermaid-js/tiny@12.2.0 (Oct 13, 2026) · last commit Oct 27, 2026

Best for: Diagrams inside docs and READMEs.

Worth knowing: Generates diagrams from Markdown-like text. A paid platform exists at mermaid.ai; its price was not found.

PlantUML

Choose from GPL, LGPL, Apache, EPL or MIT per its README. Excellent for UML and many other types, with Graphviz as its default layout engine.

Price: Free (Open-source tool; the site shows no prices and links sponsorship pages) (PlantUML pricing, checked 2026-10-23).

Platforms and license: Web, Java, Browser extension. GPL, LGPL, Apache, EPL or MIT (your choice) (open source) (license source).

Project activity: 13.4k GitHub stars · latest release v1.2026.9 (Oct 17, 2026) · last commit Oct 27, 2026

Best for: UML and architecture documentation.

Worth knowing: Repository README states it is licensed under several licenses and lets you choose. Default layout uses Graphviz.

D2

MPL-2.0 declarative diagram language with a CLI, watch mode and an online playground at play.d2lang.com; installs through Homebrew and Winget.

Price: Free (Open-source project with a free CLI and online playground; the site shows a Donate link and no paid tier) (D2 pricing, checked 2026-10-23).

Platforms and license: macOS, Linux, Windows, Web playground. MPL-2.0 (open source) (license source).

Project activity: 25.7k GitHub stars · latest release v0.9.0 (Sep 7, 2026) · last commit Oct 11, 2026

Best for: Developers who want modern text diagrams.

Worth knowing: Repo has moved to github.com/terrastruct/d2 (raw LICENSE.txt read via d2lang/d2).

Graphviz

Eclipse Public License 2.0, described on its license page as open source. Automatic graph layout from DOT text suits networks and dependency maps, not hand-tuned posters.

Price: Free (Open source; the site shows no price) (Graphviz pricing, checked 2026-10-23).

Platforms and license: Windows, macOS, Linux. EPL-2.0 (open source) (license source).

Best for: Auto-laid-out graphs and dependency maps.

Which one to pick

• Non-technical colleagues edit the diagram: draw.io (diagrams.net)
• The diagram lives in a Git repository's Markdown: Mermaid
• You draw UML: PlantUML
• You want a quick shared sketch: Excalidraw