When comparing Dia vs PlantUml, the Slant community recommends Dia for most people. In the question“What are the best flowchart / diagram softwares?” Dia is ranked 13th while PlantUml is ranked 21st. The most important reason people chose Dia is:
Has flowcharts, uml, chemical engineering, electronics, and dozens of other complete sets, built in.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Tons of icon sets
Has flowcharts, uml, chemical engineering, electronics, and dozens of other complete sets, built in.
Pro Free and open source
Pro Custom gzipped XML format
Pro Many, many integrations
For example from Emacs, Sublime Text and so on. You can learn about integrations here.
Pro Easy create diagrams from text
There is even a Visual Basic Macro to draw UML diagrams in Word.
Pro Free/libre/open source (GPL)
Source code can be browsed here.
Pro Has online editors
Such as.
Pro sphinx-contrib extension for embedding into docs
sphinx-contrib extension on PyPi
Allows you to keep your UML definitions under VCS and include them inside generated documentation with ease.
Cons
Con Unmaintained
Dia has not been updated since 2009. It's already a very complete piece of software, but still sad.
Con UX is a joke
UX is not good enough. For example, to edit element text you must double-click it, then click text box, and then you can start typing text.
Con The "plantUML-language" is a bit strange and hard to grasp for new people
Con Slow development due to it being mostly a one-man project
There’s been no recent contribution to the source forge project but from the administrator arnaud_roques. Future contribution might be difficult due to lacking code documentation.