Composable Svelte Charts

Interactive data visualization components built with Observable Plot and D3.

---

PACKAGE OVERVIEW

Package: @composable-svelte/charts

Purpose: State-driven interactive charts and data visualizations.

Technology Stack:

• Observable Plot: Declarative visualization grammar from Observable
• D3: Low-level utilities for scales, shapes, and interactions
• Motion One: Smooth transitions and animations

Chart Types:

• Scatter plots
• Line charts
• Bar charts
• Area charts
• Histograms

Interactive Features:

• Zoom & pan
• Brush selection
• Tooltips (automatic)
• Range selection
• Responsive sizing

State Management: All charts use pure reducers with type-safe actions following Composable Architecture patterns.

---

QUICK START

import { createStore } from '@composable-svelte/core';
import { Chart, chartReducer, createInitialChartState } from '@composable-svelte/charts';

// Sample data
const data = [
  { x: 1, y: 10, category: 'A' },
  { x: 2, y: 25, category: 'B' },
  { x: 3, y: 15, category: 'A' },
  { x: 4, y: 30, category: 'B' }
];

// Create chart store
const chartStore = createStore({
  initialState: createInitialChartState({ data }),
  reducer: chartReducer,
  dependencies: {}
});

// Render scatter plot
<Chart
  store={chartStore}
  type="scatter"
  x="x"
  y="y"
  color="category"
  width={800}
  height={400}
  enableZoom={true}
  enableTooltip={true}
/>

---

CHART COMPONENT

Purpose: High-level wrapper for creating charts with Observable Plot.

Props

• store: Store<ChartState, ChartAction> - Chart store (required)
• type: 'scatter' | 'line' | 'bar' | 'area' | 'histogram' - Chart type (default: 'scatter')
• width: number - Chart width (optional, responsive if omitted)
• height: number - Chart height (optional, defaults to 400px)
• x: string | ((d) => any) - X accessor (required)
• y: string | ((d) => any) - Y accessor (required)
• color: string | ((d) => any) - Color accessor (optional)
• size: number - Mark size (optional)
• xDomain: [number, number] | 'auto' - X domain (optional)
• yDomain: [number, number] | 'auto' - Y domain (optional)
• enableZoom: boolean - Enable zoom/pan (default: false)
• enableBrush: boolean - Enable brush selection (default: false)
• enableTooltip: boolean - Enable tooltips (default: true)
• enableAnimations: boolean - Enable transitions (default: true)
• onSelectionChange: (selected: any[]) => void - Selection callback (optional)

Usage

<Chart
  store={chartStore}
  type="scatter"
  x="date"
  y="value"
  color={(d) => d.category}
  size={4}
  xDomain="auto"
  yDomain={[0, 100]}
  enableZoom={true}
  enableTooltip={true}
  onSelectionChange={(selected) => console.log('Selected:', selected)}
/>

---

CHART TYPES

Scatter Plot

Purpose: Display individual data points in 2D space.

Best for: Correlations, distributions, outliers.

<Chart
  store={chartStore}
  type="scatter"
  x="temperature"
  y="sales"
  color="region"
  size={5}
  enableZoom={true}
/>

Accessories:

• x: X-axis position
• y: Y-axis position
• color: Point color (optional)
• size: Point size (optional)

Line Chart

Purpose: Show trends over time or continuous data.

Best for: Time series, trends, comparisons.

<Chart
  store={chartStore}
  type="line"
  x="date"
  y="price"
  color="ticker"
  enableZoom={true}
/>

Notes:

• Data should be sorted by X for proper rendering
• Multiple series via color accessor
• Supports missing data (gaps in line)

Bar Chart

Purpose: Compare categorical data with rectangular bars.

Best for: Category comparisons, rankings, distributions.

<Chart
  store={chartStore}
  type="bar"
  x="category"
  y="count"
  color="segment"
  enableTooltip={true}
/>

Variants:

• Vertical bars (default)
• Grouped bars (via color)
• Stacked bars (via config)

Area Chart

Purpose: Line chart with filled area below.

Best for: Cumulative data, part-to-whole relationships.

<Chart
  store={chartStore}
  type="area"
  x="date"
  y="value"
  color="category"
  enableZoom={true}
/>

Notes:

• Multiple series stack by default
• Baseline at Y=0 unless configured

Histogram

Purpose: Distribution of numerical data into bins.

Best for: Data distributions, frequency analysis.

<Chart
  store={chartStore}
  type="histogram"
  x="value"
  enableTooltip={true}
/>

Notes:

• Automatically bins data
• Y-axis shows frequency count
• Customize bins via state actions

---

STATE MANAGEMENT

ChartState Interface

interface ChartState<T = unknown> {
  // Data
  data: T[];                    // Original data
  filteredData: T[];             // After filters applied

  // Visualization config
  spec: PlotSpec;                // Observable Plot spec
  dimensions: {
    width: number;
    height: number;
  };

  // Selection
  selection: {
    type: 'none' | 'point' | 'range' | 'brush';
    selectedData: T[];
    selectedIndices: number[];
    brushExtent?: [[number, number], [number, number]];
    range?: [number, number];
  };

  // Zoom/pan
  transform: {
    x: number;
    y: number;
    k: number;  // scale factor
  };
  targetTransform?: ZoomTransform;  // For animated zoom

  // Animation
  isAnimating: boolean;
  transitionDuration: number;
}

ChartAction Types

type ChartAction<T = unknown> =
  // Data
  | { type: 'setData'; data: T[] }
  | { type: 'filterData'; predicate: (d: T) => boolean }
  | { type: 'clearFilters' }

  // Selection
  | { type: 'selectPoint'; data: T; index: number }
  | { type: 'selectRange'; range: [number, number] }
  | { type: 'brushStart'; position: [number, number] }
  | { type: 'brushMove'; extent: [[number, number], [number, number]] }
  | { type: 'brushEnd' }
  | { type: 'clearSelection' }

  // Zoom/pan
  | { type: 'zoom'; transform: ZoomTransform }
  | { type: 'zoomAnimated'; targetTransform: ZoomTransform }
  | { type: 'zoomProgress'; transform: ZoomTransform }
  | { type: 'zoomComplete' }
  | { type: 'resetZoom' }

  // Dimensions
  | { type: 'resize'; dimensions: { width: number; height: number } }

  // Config
  | { type: 'updateSpec'; spec: Partial<PlotSpec> };

Creating Initial State

import { createInitialChartState } from '@composable-svelte/charts';

const initialState = createInitialChartState({
  data: myData,
  dimensions: { width: 800, height: 400 },
  transitionDuration: 300
});

---

INTERACTIVE FEATURES

Zoom & Pan

Enable: enableZoom={true}

Controls:

• Mouse wheel: Zoom in/out
• Click + drag: Pan
• Double-click: Reset zoom

Programmatic zoom:

// Zoom in
chartStore.dispatch({
  type: 'zoom',
  transform: { x: 0, y: 0, k: 2 }  // 2x zoom
});

// Reset zoom
chartStore.dispatch({ type: 'resetZoom' });

// Animated zoom
chartStore.dispatch({
  type: 'zoomAnimated',
  targetTransform: { x: 100, y: 50, k: 1.5 }
});

Brush Selection

Enable: enableBrush={true}

Controls:

• Click + drag: Create brush
• Drag corners: Resize brush
• Drag center: Move brush
• Click outside: Clear brush

Access selected data:

const selected = $chartStore.selection.selectedData;
console.log('Selected points:', selected);

Callback:

<Chart
  store={chartStore}
  enableBrush={true}
  onSelectionChange={(selected) => {
    console.log('Selected:', selected);
    // Do something with selected data
  }}
/>

Tooltips

Enable: enableTooltip={true} (default)

Behavior:

• Hover over data points to show tooltip
• Automatically displays data values
• Tooltip content customizable via Observable Plot

Custom tooltips:

// Via Plot spec
const spec = {
  marks: [
    Plot.dot(data, {
      x: 'x',
      y: 'y',
      title: (d) => `${d.name}: ${d.value}` // Custom tooltip
    })
  ]
};

Point Selection

Enable: Click on points when enableBrush={false}

// Listen for point selection
$effect(() => {
  if ($chartStore.selection.type === 'point') {
    const selected = $chartStore.selection.selectedData[0];
    console.log('Selected point:', selected);
  }
});

// Programmatic selection
chartStore.dispatch({
  type: 'selectPoint',
  data: myDataPoint,
  index: 5
});

// Clear selection
chartStore.dispatch({ type: 'clearSelection' });

---

DATA BINDING

Static Data

const data = [
  { x: 1, y: 10 },
  { x: 2, y: 20 },
  { x: 3, y: 15 }
];

const chartStore = createStore({
  initialState: createInitialChartState({ data }),
  reducer: chartReducer,
  dependencies: {}
});

Dynamic Data Updates

// Update data
chartStore.dispatch({
  type: 'setData',
  data: newData
});

// Filter data
chartStore.dispatch({
  type: 'filterData',
  predicate: (d) => d.value > 10
});

// Clear filters
chartStore.dispatch({ type: 'clearFilters' });

Real-time Data

// Append new point
const currentData = $chartStore.data;
chartStore.dispatch({
  type: 'setData',
  data: [...currentData, newPoint]
});

// Update via Effect
Effect.run(async (dispatch) => {
  const newData = await fetchLatestData();
  dispatch({ type: 'setData', data: newData });
});

---

RESPONSIVE DESIGN

Auto-sizing

Omit width and height for responsive sizing:

<Chart
  store={chartStore}
  type="scatter"
  x="x"
  y="y"
/>

Chart will:

• Use container width (100%)
• Default height (400px)
• Resize on window resize

Fixed Dimensions

<Chart
  store={chartStore}
  type="scatter"
  x="x"
  y="y"
  width={800}
  height={600}
/>

Container-based Sizing

<div class="chart-container">
  <Chart store={chartStore} ... />
</div>

<style>
  .chart-container {
    width: 100%;
    height: 500px;
  }
</style>

Responsive Breakpoints

let chartWidth = $state(800);

$effect(() => {
  const updateWidth = () => {
    chartWidth = window.innerWidth < 768 ? 400 : 800;
  };

  window.addEventListener('resize', updateWidth);
  updateWidth();

  return () => window.removeEventListener('resize', updateWidth);
});

<Chart store={chartStore} width={chartWidth} ... />

---

ACCESSIBILITY

ARIA Labels

Chart component includes:

• role="img" - Marks as image
• aria-label - Describes chart content
• aria-describedby - Links to summary

<Chart
  store={chartStore}
  type="scatter"
  x="x"
  y="y"
  aria-label="Scatter plot showing relationship between X and Y"
/>

Screen Reader Summary

Auto-generated summary includes:

• Chart type
• Number of data points
• Selection status
• Filter status

Example output:

"Scatter plot showing 42 data points, 5 selected"

Keyboard Navigation

• Tab: Focus chart
• Arrow keys: Pan (when zoomed)
• +/-: Zoom in/out
• 0: Reset zoom
• Escape: Clear selection

---

COMPLETE EXAMPLES

Basic Scatter Plot

<script lang="ts">
import { createStore } from '@composable-svelte/core';
import { Chart, chartReducer, createInitialChartState } from '@composable-svelte/charts';

const data = [
  { x: 10, y: 20, category: 'A' },
  { x: 15, y: 35, category: 'B' },
  { x: 20, y: 25, category: 'A' },
  { x: 25, y: 45, category: 'B' }
];

const chartStore = createStore({
  initialState: createInitialChartState({ data }),
  reducer: chartReducer,
  dependencies: {}
});
</script>

<Chart
  store={chartStore}
  type="scatter"
  x="x"
  y="y"
  color="category"
  size={6}
  width={800}
  height={400}
  enableZoom={true}
  enableTooltip={true}
/>

Time Series Line Chart

<script lang="ts">
import { createStore } from '@composable-svelte/core';
import { Chart, chartReducer, createInitialChartState } from '@composable-svelte/charts';

interface DataPoint {
  date: Date;
  value: number;
  series: string;
}

const data: DataPoint[] = [
  { date: new Date('2024-01-01'), value: 100, series: 'A' },
  { date: new Date('2024-01-02'), value: 120, series: 'A' },
  { date: new Date('2024-01-03'), value: 115, series: 'A' },
  { date: new Date('2024-01-01'), value: 80, series: 'B' },
  { date: new Date('2024-01-02'), value: 95, series: 'B' },
  { date: new Date('2024-01-03'), value: 105, series: 'B' }
];

const chartStore = createStore({
  initialState: createInitialChartState({ data }),
  reducer: chartReducer,
  dependencies: {}
});
</script>

<Chart
  store={chartStore}
  type="line"
  x="date"
  y="value"
  color="series"
  width={1000}
  height={400}
  enableZoom={true}
  enableTooltip={true}
/>

Interactive Bar Chart

<script lang="ts">
import { createStore } from '@composable-svelte/core';
import { Chart, chartReducer, createInitialChartState } from '@composable-svelte/charts';

const data = [
  { category: 'Q1', revenue: 45000, expenses: 32000 },
  { category: 'Q2', revenue: 52000, expenses: 38000 },
  { category: 'Q3', revenue: 48000, expenses: 35000 },
  { category: 'Q4', revenue: 61000, expenses: 42000 }
];

const chartStore = createStore({
  initialState: createInitialChartState({ data }),
  reducer: chartReducer,
  dependencies: {}
});

let selectedCategory = $state<string | null>(null);

function handleSelection(selected: any[]) {
  selectedCategory = selected[0]?.category || null;
}
</script>

<div>
  <Chart
    store={chartStore}
    type="bar"
    x="category"
    y="revenue"
    enableBrush={true}
    enableTooltip={true}
    onSelectionChange={handleSelection}
  />

  {#if selectedCategory}
    <p>Selected: {selectedCategory}</p>
  {/if}
</div>

Real-time Data Visualization

<script lang="ts">
import { createStore, Effect } from '@composable-svelte/core';
import { Chart, chartReducer, createInitialChartState } from '@composable-svelte/charts';
import { onMount } from 'svelte';

let data = $state<Array<{ time: number; value: number }>>([]);

const chartStore = createStore({
  initialState: createInitialChartState({ data }),
  reducer: chartReducer,
  dependencies: {}
});

// Simulate real-time data stream
let intervalId: number;

onMount(() => {
  let time = 0;

  intervalId = setInterval(() => {
    const newPoint = {
      time: time++,
      value: Math.random() * 100
    };

    data = [...data.slice(-50), newPoint]; // Keep last 50 points

    chartStore.dispatch({
      type: 'setData',
      data
    });
  }, 100);

  return () => clearInterval(intervalId);
});
</script>

<Chart
  store={chartStore}
  type="line"
  x="time"
  y="value"
  yDomain={[0, 100]}
  enableAnimations={true}
/>

---

COMMON PATTERNS

Multiple Charts with Shared Selection

<script lang="ts">
const data = [...]; // Shared data

const chartStore1 = createStore({...});
const chartStore2 = createStore({...});

let selectedData = $state<any[]>([]);

function syncSelection(selected: any[]) {
  selectedData = selected;

  // Update both charts
  const indices = selected.map(d => data.indexOf(d));
  chartStore1.dispatch({ type: 'selectRange', range: [indices[0], indices[indices.length - 1]] });
  chartStore2.dispatch({ type: 'selectRange', range: [indices[0], indices[indices.length - 1]] });
}
</script>

<Chart store={chartStore1} ... onSelectionChange={syncSelection} />
<Chart store={chartStore2} ... onSelectionChange={syncSelection} />

Linked Zoom

<script lang="ts">
const masterStore = createStore({...});
const detailStore = createStore({...});

$effect(() => {
  const transform = $masterStore.transform;
  detailStore.dispatch({ type: 'zoom', transform });
});
</script>

<Chart store={masterStore} enableZoom={true} />
<Chart store={detailStore} /> <!-- Zooms with master -->

Dynamic Filtering

<script lang="ts">
let minValue = $state(0);
let maxValue = $state(100);

$effect(() => {
  chartStore.dispatch({
    type: 'filterData',
    predicate: (d) => d.value >= minValue && d.value <= maxValue
  });
});
</script>

<input type="range" bind:value={minValue} min="0" max="100" />
<input type="range" bind:value={maxValue} min="0" max="100" />
<Chart store={chartStore} ... />

---

PERFORMANCE CONSIDERATIONS

Large Datasets

Problem: Rendering 10,000+ points can be slow.

Solutions:

1. Data aggregation: Bin/group data before rendering
2. Sampling: Show subset of data (e.g., every 10th point)
3. Level-of-detail: Show more detail when zoomed in
4. WebGL rendering: Use Plot's WebGL marks (future)

// Example: Downsample data
const downsample = (data: any[], factor: number) =>
  data.filter((_, i) => i % factor === 0);

const displayData = data.length > 1000
  ? downsample(data, Math.ceil(data.length / 1000))
  : data;

chartStore.dispatch({ type: 'setData', data: displayData });

Frequent Updates

Problem: Real-time data updates cause re-renders.

Solutions:

1. Batch updates: Update every N milliseconds, not every data point
2. Sliding window: Keep fixed number of points (e.g., last 100)
3. Throttle: Limit update frequency

// Throttle updates
let pendingData: any[] = [];
let updateTimer: number | null = null;

function queueUpdate(newData: any[]) {
  pendingData = newData;

  if (updateTimer === null) {
    updateTimer = setTimeout(() => {
      chartStore.dispatch({ type: 'setData', data: pendingData });
      updateTimer = null;
    }, 100); // Update max once per 100ms
  }
}

Animation Performance

Disable animations for large datasets or frequent updates:

<Chart
  store={chartStore}
  enableAnimations={false}
  ...
/>

---

TESTING

Basic Chart Testing

import { TestStore } from '@composable-svelte/core';
import { chartReducer, createInitialChartState } from '@composable-svelte/charts';

const store = new TestStore({
  initialState: createInitialChartState({ data: [] }),
  reducer: chartReducer,
  dependencies: {}
});

// Test data update
await store.send({
  type: 'setData',
  data: [{ x: 1, y: 10 }]
}, (state) => {
  expect(state.data.length).toBe(1);
  expect(state.filteredData.length).toBe(1);
});

// Test filtering
await store.send({
  type: 'filterData',
  predicate: (d) => d.y > 5
}, (state) => {
  expect(state.filteredData.length).toBe(1);
});

Selection Testing

await store.send({
  type: 'selectPoint',
  data: { x: 1, y: 10 },
  index: 0
}, (state) => {
  expect(state.selection.type).toBe('point');
  expect(state.selection.selectedData.length).toBe(1);
  expect(state.selection.selectedIndices).toEqual([0]);
});

await store.send({ type: 'clearSelection' }, (state) => {
  expect(state.selection.type).toBe('none');
  expect(state.selection.selectedData.length).toBe(0);
});

---

TROUBLESHOOTING

Chart not rendering:

• Check Observable Plot installed: npm install @observablehq/plot
• Verify data is non-empty array
• Ensure x/y accessors match data properties

Tooltips not showing:

• Verify enableTooltip={true}
• Check Observable Plot version (0.6+ required)
• Ensure data points have valid values (not null/undefined)

Zoom not working:

• Verify enableZoom={true}
• Check chart has fixed dimensions (not 100% width/height)
• Ensure D3-zoom is installed

Poor performance:

• Reduce data points (aggregate, sample, or downsample)
• Disable animations for large datasets
• Use simpler mark types (dots vs complex shapes)

Selection not updating:

• Check onSelectionChange callback
• Verify enableBrush={true} or enableSelection={true}
• Ensure store is reactive ($chartStore.selection)

---

CROSS-REFERENCES

Related Skills:

• composable-svelte-core: Store, reducer, Effect system
• composable-svelte-components: UI components (Button, Slider, etc.)
• composable-svelte-testing: TestStore for testing chart reducers

When to Use Each Package:

• charts: 2D data visualization, interactive charts
• graphics: 3D scenes, WebGPU/WebGL (see composable-svelte-graphics)
• maps: Geospatial data (see composable-svelte-maps)
• code: Code editors, media players (see composable-svelte-code)