A largely browser based diagram and chart webapp written in Javascript. Use Ant to compile it, then zip it up and upload it to a web server. You can even use Python's BaseHTTPServer to play with it. Forces HTTPS. I was able to build it on my laptop pretty easily. Gives you the option to save to a number of different storage back-ends if you install the back-end webapp, including your local machine.
There is also an Electron webshit version for download.
Online version, free to use: https://app.diagrams.net/