RubyMine can render mermaid and plantuml diagrams directly in the Markdown preview window.

TLDR;

  1. Press ⌘ , to open the IDE settings and select Languages & Frameworks Markdown.
  2. Enable either Mermaid or PlantUML under Markdown extensions.
  3. After IntelliJ IDEA downloads the relevant extensions, click OK to apply the changes.

from JetBrains

Why does this matter?

Now that Github supports mermaid diagrams in their Markdown you can easily add all sorts of graphics to help document your codebase.

This is useful for sharing ideas about new/existing modelling using Entity Relationship Diagrams as well as Sequence Diagramming and Flowcharts.

Diagrams can quickly convey ideas and understanding to your team and your future self.

Mermaid example

erDiagram  
 CUSTOMER ||--o{ ORDER : places  
 ORDER ||--|{ LINE-ITEM : contains  
 CUSTOMER }|--|{ DELIVERY-ADDRESS : uses