[FEATURE] Add a Mosaic Plotting Widget to Preswald

[amrutha97]

Summary:
Introduce a new plotting widget that renders visualizations using [Mosaic](https://github.com/uwdata/mosaic), a high-performance declarative visualization grammar. This will expand Preswald’s charting capabilities beyond Plotly, allowing for more expressive, scalable, and Vega-Lite-compatible visualizations.

---

📌 Motivation

Preswald currently supports plotly() for visualizations. While excellent for general-purpose charts, Plotly lacks native support for declarative grammar like Vega-Lite and high-dimensional faceting. Mosaic (from UW IDL) fills this gap, enabling developers to declaratively define richly interactive and faceted plots.

Adding Mosaic support would:

• Unlock new visualization types (e.g., repeat/facet, grid layout, split axes).
• Align with Vega/Vega-Lite standards.
• Improve performance for large datasets via GPU/WebGL rendering.
• Attract data scientists and researchers familiar with Observable and Vega-Lite.

---

🧱 Proposed Implementation

1. Backend API (preswald/interfaces/components.py)

Add a new interface function:

def mosaic(
    label: str,
    spec: dict,
    size: float = 1.0,
) -> str:
    """Render a Mosaic visualization based on a Vega-Lite-compatible spec"""
    from preswald.service import PreswaldService
    import hashlib

    service = PreswaldService.get_instance()
    component_id = f"mosaic-{hashlib.md5(label.encode()).hexdigest()[:8]}"

    component = {
        "type": "mosaic",
        "id": component_id,
        "label": label,
        "spec": spec,
        "size": size,
    }

    service.append_component(component)
    return f"Mosaic: {label}"

2. Component Exposure (preswald/interfaces/__init__.py)

from .components import mosaic

3. Frontend Widget (frontend/src/components/widgets/MosaicWidget.jsx)

import React, { useEffect, useRef } from 'react';
import { Card } from '@/components/ui/card';
import { cn } from '@/lib/utils';
import { mosaic } from '@uwdata/mosaic-vega';

const MosaicWidget = ({ _label, spec, className, _size }) => {
  const containerRef = useRef(null);

  useEffect(() => {
    if (containerRef.current && spec) {
      mosaic(containerRef.current, spec);
    }
  }, [spec]);

  return (
    <Card className={cn('w-full overflow-x-auto', className)}>
      <div ref={containerRef}></div>
    </Card>
  );
};

export default MosaicWidget;

Note: Add @uwdata/mosaic-vega to frontend/package.json.

4. Dynamic Registration (frontend/src/components/DynamicComponents.jsx)

import MosaicWidget from '@/components/widgets/MosaicWidget';

case 'mosaic':
  return (
    <MosaicWidget
      {...commonProps}
      className={component.className}
      _size={component.size || 'default'}
      spec={component.spec}
    />
  );

5. Test Usage (e.g., examples/iris/mosaic_test.py)

from preswald import text, mosaic

text("# Mosaic Example")

mosaic(
    label="Iris Mosaic Plot",
    spec={
        "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
        "data": {"url": "https://vega.github.io/vega-lite/data/iris.json"},
        "mark": "point",
        "encoding": {
            "x": {"field": "sepalLength", "type": "quantitative"},
            "y": {"field": "sepalWidth", "type": "quantitative"},
            "color": {"field": "species", "type": "nominal"}
        }
    }
)

Run locally:

cd examples/iris
preswald run

---

📦 Dependencies

Update frontend/package.json:

"dependencies": {
  "@uwdata/mosaic-vega": "^0.2.0"
}

---

✅ Acceptance Criteria

• Mosaic widget renders Vega-Lite-style JSON specs.
• Users can pass spec via Python and render rich charts.
• Compatible with browser and server execution.
• Shipped with an Iris example in /examples/iris/mosaic_test.py.

---

📘 References

• [Mosaic GitHub](https://github.com/uwdata/mosaic)
• [Mosaic Vega Docs](https://uwdata.github.io/mosaic/)
• [Preswald Custom Component Guide](https://docs.preswald.com/addnewcomponent)

[Rupak182]

@amrutha97 I couldn't find uwdata/mosaic-vega library., So I have used vega-embed instead. Should I change the name of widget and component based on that.