There are many alternatives to Dia for Linux if you are looking for a replacement. The best Linux alternative is diagrams.net, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Dia and many of them is available for Linux so hopefully you can find a suitable replacement. Other interesting Linux alternatives to Dia are yEd Graph Editor (Free), LibreOffice - Draw (Free, Open Source), TreeSheets (Free, Open Source) and PlantUML (Free, Open Source).
Gaphor is a Python-based, easy to use modeling environment. This means that you are able to create nice UML diagrams for documentation and to assist you with design decisions. Gaphor will help you create your applications.
Core2D is a multi-platform application for making data driven 2D diagrams. The application has built-in wyswig vector graphics editor where you can bind data to shapes, share data across multiple documents, edit documents using layers.