Dia is not available as a web app but there are some alternatives that runs in the browser with similar functionality. The best open source Online alternative is Excalidraw. If that doesn't suit you, our users have ranked more than 50 alternatives to Dia and eight of them are open source and available as a web app so hopefully you can find a suitable replacement. Other interesting open source Online alternatives to Dia are PlantUML, quiver diagram editor, Kroki and Wireflow.
Excalidraw is an open-source whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them. Available directly online as a web app and also through CodeSandbox to easily see how the code works, and the app is available for self-hosting.
PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams. The generator can be run anywhere within JVM and integrated with various application such as wiki, text editor, IDE, programming language, documentation generator, and others.
PlantUML has no way to make a diagram or chart without knowing command line and learning a whole new markup language.
Some users think PlantUML is a great Dia alternative, some don't.
Efficient web-based tool for constructing high-quality commutative diagrams. Offers LaTeX export with embedded links, intuitive interface supporting both mouse and keyboard inputs, and features like multiple selections, macro support, and smart label alignment.
Creates diagrams from textual descriptions! It provides a unified API with support for BlockDiag, BPMN, Bytefield, C4 (with PlantUML), Ditaa, Erd, GraphViz, Mermaid, Nomnoml, PlantUML, SvgBob, UMLet, Vega, Vega-Lite, WaveDrom.
Feature-rich, open-source Wireflow offers a collaborative platform for online creation of complex user flow prototypes. Ideal for team work in UI design, wireframing, and mind mapping, promoting idea sharing in real-time environment aided by modern whiteboard features.
Diagramo is an online flowchart software
Intuitive interface, nice rendering, web-based so can be used both locally and on a website
PathVisio is a free open-source pathway analysis and drawing software that allows drawing, editing and analysing biological pathways.
It's an OpenSource WhiteBoard app with very useful features ("magnets"). There's a running instance at github.io. You can save & reopen your boards...
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Not a GUI diagramming tool. By this logic graphviz is an alternative to Dia.