The best alternatives to DiaChat are Whiteboard.fm, DigStack, and UMLBoard. If these 3 options don't work for you, we've listed a few more alternatives below.
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UMLBoard is a lightweight and easy-to-use graphical UML class designer for creating nice hand-drawn-looking UML class diagrams. It provides an intuitive user interface with full inline-editing capabilities and different image export options for your diagrams.
Gliffy is software for diagramming via an HTML5 cloud-based app. It is used to create UML diagrams, floor plans, Venn diagrams, flowcharts and various other kinds of diagrams online. Gliffy diagrams can be shared with and edited by users in real time.