hive.chart-creation-foundations

# Chart creation foundations

These tools render BI/financial-analyst-grade charts and diagrams that show up live in the chat AND save as high-DPI PNGs in the user's queen session dir.

## The embedding contract — one rule

> **To put a chart in chat, call `chart_render`. The chat reads `result.spec` and renders the chart live in the message bubble. The download link is `result.file_url`. Do not write `![chart](...)` image markdown by hand — the tool's result drives the UI.**

That's it. One tool call, one chart in chat, one file on disk. No two-step "remember to also save it" pattern. The chat's chart-rendering UI is fed by the tool result envelope automatically.

## When to chart at all

Chart when the data is **visual at heart**: trends over time, distributions, comparisons across categories, hierarchies, flows, geo. Skip the chart when:

- The point is one number → just say it. ("Revenue was $4.2M, up 12% YoY.")
- The point is a ranking of 5 things → use a markdown table with bold and emoji indicators.
- The data is so noisy a chart would mislead → describe the takeaway in prose.

A chart costs the user attention. It must repay that cost with a takeaway they couldn't get from prose.

## ECharts vs Mermaid — the picking rule

| Use ECharts (`kind: "echarts"`) when... | Use Mermaid (`kind: "mermaid"`) when... |
|---|---|
| You're plotting **numbers over categories or time** | You're showing **structure, not data** |
| Bar / line / area / scatter / candlestick / heatmap / treemap / sankey / parallel coordinates / calendar / gauge / pie / sunburst / geo map | Flowchart / sequence / gantt / ERD / state diagram / mindmap / class diagram / C4 architecture |
| The viewer's question is "how much / how many / what's the trend" | The viewer's question is "what calls what / what depends on what / what happens after what" |

If both fit (rare), prefer ECharts — its rasterized output is a proper data chart for slides; Mermaid's diagrams are for technical docs.

## The aesthetic baseline (non-negotiable)

These are the rules that turn an Excel-default chart into a Tableau-grade one. Every chart you produce must follow them.

### 1. Theme & background
- `chart_render` has **no `theme` parameter**. The renderer reads the user's UI theme from the desktop env (`HIVE_DESKTOP_THEME`) so the saved PNG matches what the user is actually looking at. You don't pick; the system does.
- Title goes in `option.title.text`, NOT in the message body. The chart is self-contained.

### 2. Palette discipline — DO NOT set `color` on series

The OpenHive ECharts theme is auto-applied to every `chart_render` call. It defines:
- An 8-hue **categorical palette** for multi-series charts (honey orange, slate blue, sage, terracotta, bronze, indigo, olive, rust)
- Cozy spacing (`grid.top: 90`, `grid.bottom: 56`, etc.)
- Brand typography (Inter Tight)
- Tasteful axis lines + dashed gridlines

**Do not set `option.color`, `option.title.textStyle`, `option.grid`, or `option.itemStyle.color` on series.** The theme covers it. If you do override, you'll fight the brand palette and the chart will look generic.

When you need data-encoded color (NOT category color):
- **Sequential** (magnitude): use `visualMap` with `inRange.color: ['#fff7e0', '#db6f02']` (light-to-honey)
- **Diverging** (positive/negative): use `visualMap` with `inRange.color: ['#a8453d', '#f5f5f5', '#3d7a4a']` (terracotta/neutral/sage)
- **Semantic up/down** (candlestick is auto-themed): for explicit gain/loss bars use `#3d7a4a` (gain) and `#a8453d` (loss), NOT `#27ae60` / `#e74c3c`.

### 3. Typography
The default font (`-apple-system, "Inter Tight", system-ui`) is already wired in the renderer — don't override unless the user asked. Set `option.textStyle.fontSize: 13` for body labels, `16` for axis names, `18` bold for the title.

### 4. No chartjunk
- **No 3D**. Ever. 3D pie charts and 3D bar charts are visual lies.
- **No drop shadows** on bars or lines. The default flat ECharts look is correct.
- **No gradient fills** unless the gradient encodes data (e.g. heatmap fill).
- **No neon colors**. Saturation belongs on highlighted bars, not on every series.
- **No more than 5 stacked colors** in a stacked bar — past that the eye can't separate them.

### 5. Axis hygiene
- X-axis labels rotate 45° only when they overflow. Otherwise horizontal.
- Y-axis starts at 0 for bar/area charts (truncating misleads). Line charts can start at min - 5%.
- Use `option.yAxis.axisLabel.formatter: '{value} M'` to add units, NOT a separate "USD millions" subtitle.
- Date axes: pass ISO strings (`"2024-01-15"`) and ECharts handles the layout. Use `xAxis.type: "time"`.

### 6. One message per chart
Every chart goes in its own assistant message (or its own `chart_render` call). Do not pile 4 charts into one wall of tool calls — the user can't focus and the chat gets noisy.

## Calling `chart_render` — the canonical pattern

```
chart_render(
  kind="echarts",
  spec={
    "title": {"text": "Q4 revenue by region", "left": "center"},
    "tooltip": {"trigger": "axis"},
    "xAxis": {"type": "category", "data": ["NA", "EU", "APAC", "LATAM"]},
    "yAxis": {"type": "value", "axisLabel": {"formatter": "${value}M"}},
    "series": [{"type": "bar", "data": [12.4, 8.7, 5.3, 2.1], "itemStyle": {"color": "#db6f02"}}]
  },
  title="q4-revenue-by-region",
  width=1600, height=900, dpi=300
)
```

Returns:
```
{
  "kind": "echarts",
  "spec": {...echoed...},
  "file_path": "/.../charts/2026-04-30T...q4-revenue-by-region.png",
  "file_url": "file:///.../q4-revenue-by-region.png",
  "width": 1600, "height": 900, "dpi": 300, "bytes": 142318,
  "title": "q4-revenue-by-region", "runtime_ms": 287
}
```

The chat panel reads `result.spec` and mounts ECharts in the message bubble. The user sees the chart immediately. The PNG is on disk and the chat shows a download link from `result.file_url`. **You don't write that link — it appears automatically.**

## The 12 chart types you'll use 95% of the time

| When |