https://knsv.github.io/mermaid/

place the <$mermaid> widget and then use the mermaid syntax with in the text.

<$mermaid text="
        graph LR
            A-->B
            B-->C
            C-->A
            D-->C
"><$mermaid>

An alternative method is to make the Tiddler type to be text/vnd.tiddlywiki.mermaid. You can also try putting the mermaid syntax within the <$mermaid> element, but this does not work as well due to TW5 parsing.

You can install by browsing to https://gt6796c.github.io/ and dragging the link to your TiddlyWiki.