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.

7. Notes and caveats live outside the plot

The theme fixes the grid, so there is no free space inside the chart for extra text. Text you add there lands on top of the axis name or the tick labels.

• One short qualifier (under ~90 characters) can go in option.title.subtext. Never a second subtext line.
• Do not add graphic text elements for footnotes, sources or exclusions. Put those in the message that carries the chart.
• To flag a qualified data point, mark its category label ("Supabase*") and explain the asterisk in the message.
• Show a missing value as a gap with (N/A) in its category label, not as a zero-length bar.

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	ECharts type	Notes
Trend over time	series.type: "line"	Smooth = smooth: true only when data is noisy
Multi-metric trend	Two line series with yAxis: [{}, {}]	Separate scales when units differ
Category comparison	series.type: "bar"	Sort by value descending, not alphabetically
Stacked composition	bar with stack: "total"	Cap at 5 categories
Distribution	series.type: "boxplot" or bar of bins	Boxplot for ≥3 groups; histogram for one
Two-variable correlation	series.type: "scatter"	Add regression markline if relevant
Candlestick / OHLC	series.type: "candlestick"	Date axis + dataZoom range slider
Geo distribution	series.type: "map"	Bundled world and country GeoJSONs
Hierarchy / share	series.type: "treemap" or sunburst	Use treemap for >12 leaves; pie only for 2-5
Flow	series.type: "sankey"	Names matter — keep them short
Calendar density	series.type: "heatmap" + calendar	Daily metrics over a year
KPI scorecard	series.type: "gauge"	Set min, max, threshold band

Worked specs for each are in references/ — paste, modify, render.

Mermaid quick rules

chart_render(
  kind="mermaid",
  spec="""
flowchart LR
  A[Customer signs up] --> B{Onboarded?}
  B -- yes --> C[Activate trial]
  B -- no --> D[Email reminder]
""",
  title="signup-flow"
)

• One diagram per chart_render call.
• Keep node labels short (≤20 chars).
• Use flowchart LR for left-to-right; TD for top-down. LR reads better in a chat bubble.
• For sequence diagrams, indicate async with ->> (open arrow) and sync return with -->> (dashed).
• Don't try to encode data in mermaid (no widths, no quantities) — that's an ECharts job.

Common mistakes the agent makes

1. Writing ![chart](file://...) markdown by hand. Don't. The chat renders from the tool result automatically. Manual image markdown will display nothing (file:// is blocked from arbitrary chat content).
2. Calling chart_render twice for the same chart "to embed and to save". Only one call. The single call does both.
3. Overriding fonts to fancy display faces. Stay with the default; the agent's job is data, not typography.
4. Pie charts with 12 slices. Use a horizontal bar chart sorted by value. Pie is only for 2-5 mutually-exclusive shares.
5. Forgetting axisLabel.formatter for currency / percentage. A y-axis showing "12000000" is unreadable; "12M" is correct.
6. Putting a chart's title in the message body. Set option.title.text instead so the title is part of the saved PNG.