Main "killer" features for me are:

- d2 is a standalone executable compiler, I once tried mermaid-cli (mmdc) but couldn't get it to work properly plus anything I need to install with npm scares the hell out of me

- ASCII rendering: I love rendering to ASCII which I can copy-paste around.

But I do use mermaid a lot embedded in other programs (e.g Obisidian). The selection of different diagram types is amazing.

I use d2 in obsidian fwiw

Same, I spend an inordinate amount of time in mermaid, and I just can't see a reason for D2 over Mermaid especially when you can write a single Markdown doc and jump in between code, prose and diagrams as simply as

```mermaid ```

```typescript ```

I think if we could somehow use such tools to navigate large codebases visually. That would be a big plus.

This is Markdown with embedded Mermaid with Markdown inside the box named foo. In VS Code, links are opened in editor. In Jetbrains it unfortunately opens in browser.

    # A Simple Diagram
    
    ```mermaid
    graph TD;
    foo["<b>bold</b> <a href='./pyproject.toml'>./pyproject.toml</a>"]
    ```

We could ask LLMs to generate module diagrams with links.

What does that killer program need to do?

I don't know exactly but none of the diagram building languages I've used have been a great experience. I guess they just feel "rough in the hands" somehow to me. There's always some point of frustration I get to with the layout systems. They're essential for quickly visualizing graph structures and such but even smaller hand authored ones end up feeling unwieldy too. Can't put it in to works but it feels like there could be a major improvement beyond what even D2 studio offers, when it comes to the language and workflow around it.

I feel similarly about charting libraries.

> I guess they just feel "rough in the hands" somehow to me.

I'd be curious to know if Ilograph is among the languages you've tried and if you feel the same way (I'm the author). Making it feel good to edit (the opposite of "rough in the hands"?) is an explicit goal, and to that end there is an IDE with context-aware autocomplete.

Ilograph looks pretty neat! I'll have to try it

doesn't seem to be open source

It's not, no