H1Adding a visualURL copied

Four steps. The registry drives the fence names, the declared capabilities, and this folder, so nothing else needs editing.

1. Write the renderer in vyasa/extensions_builtin/visuals/render.py. It takes (rows, attrs) and returns HTML carrying classes only.
2. Add one register(Visual(...)) entry in registry.py: name, summary, cell_names, the renderer, its options, and one example.
3. Style it in static/visuals.css using the --tone variable, so every theme and both colour schemes follow without further work.
4. Add a case to tests/test_visuals.py and a demo page to this folder. A test asserts every registry entry has one, so step 4 is enforced.

H2ConstraintsURL copied

• cell_names maps positional cells onto field names. bar uses ("label", "value"); card uses ("value", "label", "note"), because that is the order a reader scans them.
• Never emit an inline style except a computed width or height. Colour belongs in the stylesheet.
• Escape every author string with escape_attr.
• Raise VisualError for anything the author can fix. It becomes a visible card — see errors.

H2When not to add oneURL copied

If the visual needs nested data or a grammar of its own, it does not belong in the registry: the shared row grammar would become a lie. Give it its own extension, as Vega-Lite has.

That is the line. card, bar and stack all read label and value. A treemap, a flame graph or a sankey needs a tree or an edge list, so each would be its own extension rather than a registry entry.