A slightly older cousin of Mermaid for drawing Diagrams using Code.
PlantUML is a Diagrams as Code platform for drawing 20+ kinds of diagrams using fairly simple, fairly intuitive (although, arguably less intuitive than Mermaid) syntax. See PlantUML vs Mermaid for more on that.
PlantUML was built primarily for use locally - its documentation starts with information about how to install PlantUML locally, but has been adapted into at least one web-based application:
Example Syntax
@startuml
Bob -> Alice : hello
Alice -> Bob : oh hi.
Bob -> Charlie : I was talking with you.
Charlie -> Alice : Sorry, he's just like that
Alice -> Alice : I guess I'll go
@enduml
Source
Source
Open-source tool that uses simple textual descriptions to draw beautiful UML diagrams.