I looked for a good Linux-based alternative to Visio for a long time, but I was never very happy with any of the options. I stopped looking once I started using Draw.io a few years ago. It integrates with Google Docs where I’m doing much of my documentation these days, and the drawings look great.
I’m reworking the publishing toolchain I’ve used to write and publish two ebooks and trying to get it released as a proper Rubygem - NerdPress.
It’s still not ready for prime time, but I’d love to hear from anyone who could use something like this.