If you have striggled a lot to find good diagram makers/editor. Here is the one.

draw.io is free online diagram software for making flowcharts, process diagrams, org charts, UML, ER and network diagrams.

Try it. Its open source. Even the code is open source, you can get the war and run it in you tomcat offline...

I am listing few type of diagram you can draw are

1. ER Diagram
2. UML Diagram
3. Business process workflows
4. Bootstrap components for mock screens
5. Wireframes
6. Floor plan
7. Network diagram

Many more...

Explore!!!

https://www.draw.io