ECharts Backend

Compiles the core semantic layer into Apache ECharts option objects. Uses a series-based data model rather than VL's encoding-channel approach.

Output Format

jsonc

{ "xAxis": {...}, "yAxis": {...}, "series": [{ "type": "bar", "data": [...] }], "tooltip": {...}, "legend": {...}, "grid": {...} }

An ECharts option object with _width/_height hints and optional _warnings. Consumed by echarts.init(dom).setOption(spec).

Assembly Pipeline

Phase	Step	Description
0	`resolveSemantics`	Shared — resolve field types, aggregates, sort orders
0a	`declareLayoutMode`	Template layout declaration
0b	`convertTemporalData`	Shared — temporal parsing
0c	`filterOverflow`	Shared — category truncation
1	`computeLayout`	Shared — step sizes, subplot dimensions
2	Build `resolvedEncodings` → `template.instantiate` → `ecApplyLayoutToSpec` → `ecCombineFacetPanels` → tooltips	Final ECharts option

Key difference from VL: No buildVLEncodings step. ECharts templates read channelSemantics directly and produce series/axis config themselves.

File Structure

echarts/
  assemble.ts          – assembleECharts(): Phase 2 assembly
  instantiate-spec.ts  – ecApplyLayoutToSpec(), ecApplyTooltips()
  facet.ts             – ecCombineFacetPanels(): synthetic multi-grid faceting
  index.ts             – barrel exports
  templates/
    index.ts           – template registry (13 templates, 6 categories)
    scatter.ts         – Scatter Plot
    bar.ts             – Bar, Grouped Bar, Stacked Bar
    line.ts            – Line Chart
    area.ts            – Area, Streamgraph
    pie.ts             – Pie Chart
    histogram.ts       – Histogram
    heatmap.ts         – Heatmap
    boxplot.ts         – Boxplot
    candlestick.ts     – Candlestick Chart
    radar.ts           – Radar Chart
    rose.ts            – Rose Chart
    streamgraph.ts     – Streamgraph (themeRiver)
    utils.ts           – shared utilities

Template Definitions (13 templates)

Category	Charts
Scatter & Point	Scatter Plot, Boxplot
Bar	Bar Chart, Grouped Bar, Stacked Bar, Histogram, Heatmap
Line & Area	Line Chart, Area Chart, Streamgraph
Part-to-Whole	Pie Chart
Financial	Candlestick Chart
Polar	Radar Chart, Rose Chart

Known Issues & Notes

Custom faceting module (facet.ts, 252 lines): ECharts has no native faceting, so this module synthesizes multi-grid layouts with grid[], xAxis[], yAxis[], series[] (indexed), and graphic[] for header labels. Only axis-based charts support faceting — pie, radar, and themeRiver do not.
Bar sizing uses explicit barWidth, barCategoryGap, barGap pixel values rather than VL's declarative step sizing.
Zero-baseline uses axis.min / scale: true instead of VL's scale.zero.
Axis-less charts (pie, radar) are detected separately in instantiate-spec.ts — they skip axis/grid config entirely.
ECharts Streamgraph uses themeRiver series type, which has different data shape requirements.
The ecCombineFacetPanels step is unique to ECharts — it converts separate facet panel specs into a single multi-grid option.