--- name: layerchart description: Expert guide for LayerChart, a Svelte component library for building diverse data visualizations (Cartesian, radial, hierarchical, geo, graph) with unopinionated building blocks, motion primitives, and advanced interactions. license: MIT --- # LayerChart Skill LayerChart is a comprehensive Svelte visualization framework built on Layer Cake, providing composable components for creating responsive, interactive charts across multiple visualization types. ## Core Architecture LayerChart operates on a component-based, data-driven philosophy. The library provides three primary categories of components: **Data-Driven Components** render visual marks directly from data (Area, Bars, Spline, Pie, Sunburst, Treemap, Sankey, etc.). These components automatically handle scale transformations through LayerCake's context. **Motion-Enabled SVG Primitives** (Rect, Circle, Arc, Group, Line, Path) provide low-level drawing utilities with built-in Svelte transition support for animated data updates. **Utility Components** handle legends, tooltips, pan/zoom interactions, annotations, and layout operations (hierarchy, geo projections). ## Visualization Types - **Cartesian**: Bar, Area, Stack, Scatter, Histogram, ClevelandDotPlot, BarStack, BarGroup - **Radial**: Pie, Arc, Sunburst, Threshold - **Hierarchical**: Pack, Tree, Treemap, Partition - **Graph**: Sankey, Link, Graph utilities - **Geographic**: Choropleth, Spike Map, Bubble Map, GeoTile, GeoPath, StateMap, AnimatedGlobe, Globe projections (Mercator, Azimuthal, Equal Earth, etc.) ## Key Patterns ### Data Preparation Use LayerChart's data transformation utilities before passing to visualizations: - `stack()` - converts wide-format data into stacked series with baseline/top values - `bin()` - groups data into histogram bins with x0/x1 bounds - `groupLonger()` - pivots wide-format to long-format (one row per value) - `flatten()` - flattens nested arrays one level, with optional accessor - `calcExtents()` - calculates min/max across multiple fields, skipping nulls ### Component Composition All LayerChart visualizations sit within a LayerCake wrapper that establishes scales and context. Child components access scales via Svelte's context API. ```svelte

``` ### Interaction Patterns - **Tooltips**: Position over data with snap-to-data options - **Pan/Zoom**: Built-in context utilities for interactive navigation - **Highlighting**: Hover states trigger visual emphasis (opacity, stroke changes) - **Selection**: Use reactive variables and event handlers for interactive filtering ### Responsive Design LayerChart automatically handles responsive layouts via `padding` configuration and container dimensions. Components reactively update when data or scales change. ## Common Implementation Tasks **Bar Charts**: Use `Bars` component with `x` as categorical field. Stack with `BarStack` or group with `BarGroup` for multi-series. **Time Series**: Configure `xScale={scaleTime()}` with temporal data. Use `AxisX` with `tickFormat` for readable date labels. **Geographic Visualizations**: Select appropriate projection (Mercator for web maps, Azimuthal for polar), use `GeoPath` for boundaries, `Choropleth` for value mapping. **High-Volume Data**: Render marks via Canvas instead of SVG for 5000+ points. Layer SVG axes/legends with Canvas for hybrid rendering. **Stacked/Grouped Series**: Use `stack()` utility to transform data, then render via `AreaStack`/`BarStack` components. ## Performance Considerations - Canvas rendering for 5000+ points (~60fps on modern hardware) - SVG for interactive elements and animations (<500 points recommended) - Hybrid approach: Canvas for marks + SVG for axes/legends - Scale calculations are reactive—only update scales when data/domain changes - Memoize expensive data transforms outside component lifecycle ## Styling and Customization All primitive components support standard SVG/Canvas attributes (stroke, fill, opacity, strokeWidth). Use Svelte's reactive statements for conditional styling based on interaction state or data values. Gradient fills, patterns, and clipping available via `ClipPath`, `RectClipPath`, `CircleClipPath` components with SVG ``. ## Integration Notes - Works seamlessly with D3 scales (linear, time, ordinal, log, threshold) - Supports multiple render contexts in same chart (SVG + Canvas + HTML) - Fully accessible with ARIA attributes on SVG elements - SSR-compatible for server-side rendering in SvelteKit - Zero external dependencies beyond Svelte and d3-array utilities