That being said, if you want this to be useful for people in general, not just programmers:
- People like WYSIWIG. Markdown and the split-pane view seems to be something only programmers like. So I'd suggest being able to do everything, or mostly everything, directly in the rendered/HTML panel. (Maybe the Markdown panel is for power users only)
- This is great for working with individual calculations, but a lot (most?) of spreadsheet use is about applying formulas to whole rows, columns, and tables of values. I see you support basic tables, but they're a huge pain to encode/format/edit in Markdown, and I don't see any ability to support things like 200 rows x 5 columns and do things like calculate sums and averages
So I think there's a ton of potential here! But I think WYSIWIG and easy tabular data support are going to be key here for broader usage. While the kinds of programmers this seems aimed at now, are already using Jupyter notebooks and Matplotlib for this kind of thing. (Like, when you describe "why did I build this", I don't understand why you didn't just fire up a Google Colab notebook.)