See if you need Mermaid plugin

I just want to test how to draw stuff on the github wiki

Not so happy with uploading a static file:

Mermaid drawing from https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/ in a md file

If this:

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

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is text and not a drawing, perhaps you need a plugin from the list here: https://stackoverflow.com/questions/53883747/how-to-make-github-pages-markdown-support-mermaid-diagram/69750803#69750803