PlantUML is a tool to create diagrams by writing code in a declarative language. When such code is published on a website, it needs a server to generate images from a code.
Unfortunately, it’s not 
open source 
PlantUML is a tool to create diagrams by writing code in a declarative language. When such code is published on a website, it needs a server to generate images from a code.
Unfortunately, it’s not open source