There are a number of other PlantUML tools that are make to work offline, so you may want to check those out. There are Desktop versions many of which I have used and they work well. Take a look at this website … http://plantuml.com/running. Look under the header “Other” for “PlantUML Editor”. These all use the same common PlantUML language.
However, I think you could probably use PlanText.com as a standalone (at least the main features) if you install a local PlantUML server and then change the server that PlantText points to under the Settings button. This is a bit complicated if you are not technical. But, I will try this out and let you know how it works. The instructions for setting up the local PlantUML server are here … http://plantuml.com/server … if you want to try it now.
I’ll post something on this thread once I have time to test it out. Cheers.