Include diagrams in your Markdown files with Mermaid. https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/

github-markdown-with-mermaid.md

Class Diagram

classDiagram
    Animal <|-- Duck
    Animal <|-- Fish
    Animal <|-- Zebra
    Animal : +int age
    Animal : +String gender
    Animal: +isMammal()
    Animal: +mate()
    class Duck{
        +String beakColor
        +swim()
        +quack()
    }
    class Fish{
        -int sizeInFeet
        -canEat()
    }
    class Zebra{
        +bool is_wild
        +run()
    }

ER Diagram

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

Gantt Chart

gantt
       dateFormat  YYYY-MM-DD
       title Adding GANTT diagram functionality to mermaid

       section A section
       Completed task            :done,    des1, 2014-01-06,2014-01-08
       Active task               :active,  des2, 2014-01-09, 3d
       Future task               :         des3, after des2, 5d
       Future task2              :         des4, after des3, 5d

       section Critical tasks
       Completed task in the critical line :crit, done, 2014-01-06,24h
       Implement parser and jison          :crit, done, after des1, 2d
       Create tests for parser             :crit, active, 3d
       Future task in critical line        :crit, 5d
       Create tests for renderer           :2d
       Add to mermaid                      :1d

       section Documentation
       Describe gantt syntax               :active, a1, after des1, 3d
       Add gantt diagram to demo page      :after a1  , 20h
       Add another diagram to demo page    :doc1, after a1  , 48h

       section Last section
       Describe gantt syntax               :after doc1, 3d
       Add gantt diagram to demo page      :20h
       Add another diagram to demo page    :48h

Flowchart

graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;

Sequence Diagram

sequenceDiagram
    Alice->>John: Hello John, how are you?
    John-->>Alice: Great!

Reference

• Official Announcement: Include diagrams in your Markdown files with Mermaid | The GitHub Blog
• mermaid - Markdownish syntax for generating flowcharts, sequence diagrams, class diagrams, gantt charts and git graphs.
• ProTip/mermaid記法でガントチャートを描く - docs.esa.io