# Chart Theming — Enhancement Scope

> **Status:** scope consolidated 2026-06-12. Single forward-looking home for chart-theme
> work: separating the opinionated house style from the legibility minimum, a preview
> theme selector, config merge/extract, custom named themes, and fonts (shipped roster +
> user-loaded). Read against `src/core/vega-themes.ts`,
> `src/app/services/chart-renderer.ts`, and `docs/architecture/05` §3.
>
> **Goal (the brief):** "here's how you can easily transform your Vega-Lite charts to not
> look like stock Vega-Lite charts" — make the house style one option among several,
> let a user apply custom branding (colors **and fonts**) quickly, and keep every byte
> self-hosted and offline-capable.

---

## 1. Where we stand (the audit)

Everything opinionated lives in **one file** — `src/core/vega-themes.ts` — injected as
vega-embed's `config` option at embed time (`chart-renderer.ts`). It is never baked into
the stored spec; pasting a snippet into the Vega editor renders stock. No CSS reaches
into the chart DOM. Exports (PNG/SVG) render through the same view, so they **carry the
theme**.

Merge precedence (verified in `vega-lite/src/compile/compile.ts`):
`mergeConfig(opt.config, spec.config)` — **the spec's own `config` wins** over our
injected theme, property by property. A snippet can already opt out of any of it.

Exact diff vs. stock Vega-Lite (defaults read from `vega-parser/src/config.js`):

| Property                   | Astrolabe (light / dark)    | Stock Vega-Lite      |
| -------------------------- | --------------------------- | -------------------- |
| `background`               | transparent                 | `white`              |
| `font`                     | IBM Plex Sans stack         | `sans-serif`         |
| `title`                    | 16px / 600 / app text color | 13px / bold / black  |
| `axis.domainColor`         | `#c6c6c6` / `#525252`       | `#888`               |
| `axis.gridColor`           | `#e0e0e0` / `#393939`       | `#ddd`               |
| `axis.gridDash`            | `[2,2]`                     | solid                |
| `axis.labelColor`          | `#525252` / `#a8a8a8`       | black                |
| `axis.titleColor`          | `#161616` / `#f4f4f4`       | black                |
| `axis.label/titleFontSize` | 11 / 12                     | 10 / 11              |
| `axis.titleFontWeight`     | 600                         | bold (700)           |
| `range.category`           | Carbon data-viz 14-color    | tableau10 (10-color) |
| `view.stroke`              | transparent                 | `#ddd` plot border   |

Untouched: everything else — notably the **default mark color stays Vega blue
`#4c78a8`**; the Carbon palette only kicks in once a color encoding exists.

The config splits into two layers with different standing:

- **Base (legibility/integration)** — required for charts to be readable on our panes at
  all, dark mode especially: `background: transparent` + the guide _colors_ (stock black
  text on a dark pane is illegible). Structurally the same job `[data-theme]` does for
  the rest of the app.
- **Expressive (house style)** — genuinely opinionated: Plex, the Carbon categorical
  palette, dotted grid, bumped guide sizes/weights, 16px title, no plot border. Strip it
  and charts still work in both UI themes; they just look like Vega-Lite.

## 2. What vega-editor does (and what we take)

Read from the local clone (`reference/vega-editor`, `components/config-editor/`):

- **Theme dropdown = the `vega-themes` npm package** (~14 preset configs: excel,
  ggplot2, fivethirtyeight, latimes, powerbi, googlecharts, urbaninstitute, dark, four
  Carbon themes) + a `custom` sentinel. Already in our tree — vega-embed depends on it.
- **Picking a theme is a one-shot copy** of the preset JSON into a config editor pane;
  any hand-edit flips back to `custom`. No live binding.
- **The config pane** feeds `opt.config` at compile — the slot we already use. Two
  Monaco context-menu commands bridge pane ↔ spec: **Merge Config Into Spec** (pane →
  `spec.config`, spec's existing keys win, pane empties) and **Extract Config From
  Spec** (the inverse).
- **No custom-theme saving.** One global localStorage state blob; `custom` is "whatever
  is in the pane". Nothing to borrow for named themes — that part is our own design.

The structural mismatch: vega-editor is a scratchpad for one transient document;
Astrolabe is a library. **Decided 2026-06-12:** theme choice is **not per-snippet** —
`spec.config` _is_ the per-snippet mechanism, and merge/extract makes it ergonomic. The
app-level selector is a global preference.

## 3. Fonts (researched 2026-06-12)

**The hard constraint:** `vega-scenegraph/src/util/text.js` measures every label via
canvas `measureText` **regardless of renderer**. A font that finishes loading after
embed leaves the whole layout measured with fallback metrics. Any custom-font path must
`await document.fonts.load('<weight> 11px "Family"')` per used face **before**
`renderSpec`. Once loaded, SVG view, canvas view, and PNG export all work for free.
**Known limitation:** SVG _export_ carries only the family name — a viewer without the
font sees fallback (industry standard; data-URI `@font-face` embedding is a heavy
maybe-later).

**Shipped roster (self-hosted, no CDN — same `@fontsource` mechanism as Plex).**
Measured latin woff2 sizes (jsdelivr, 2026-06-12): regular text faces run **13–25KB per
weight**; handwriting (Caveat) ~50KB. A ~9-family roster at ~2 weights ≈ **400–450KB
latin**. All-subsets multiplier ≈ 3–4× (Inter: 87KB all-subsets vs 23KB latin, one
weight). Current dist is 7.8MB with 412KB of Plex — the roster roughly doubles font
payload; acceptable.

Candidate roster (final pick deserves a visual specimen pass, not a chat decision):

| Role                  | Faces (weights)                                                                                       |
| --------------------- | ----------------------------------------------------------------------------------------------------- |
| Already shipped, free | IBM Plex Sans, IBM Plex Mono                                                                          |
| Dataviz sans          | Inter (400/600), Roboto Condensed (400/600), Libre Franklin (400/600)                                 |
| Brand-coherent        | IBM Plex Serif (400/600), IBM Plex Sans Condensed (400/600)                                           |
| Editorial serif       | Source Serif 4 _or_ Spectral (400/600)                                                                |
| Exotic / display      | Space Grotesk (400/600), Playfair Display (400/700), Caveat (400/600, "sketch"), Space Mono (400/700) |

**Subset/precache strategy:** chart fonts are decoration with automatic per-glyph
fallback (`unicode-range`), not app capability — non-latin data labels falling back to
the system font is degraded styling, not a broken app (contrast the Plex Cyrillic
lesson, which was UI capability). Plan: **ship all subsets in dist** (~1.2–1.5MB dist
growth), **precache latin only** (~+400KB), runtime-cache the remaining subsets
same-origin (CacheFirst) so a used subset persists offline after first render.

**User-loaded fonts (the branding case — primary).** Real brand fonts are licensed and
usually _not_ on Google Fonts. Path: upload woff2/ttf → bytes in IndexedDB (the
datasets persistence pattern) → `new FontFace(family, bytes)` + `document.fonts.add()`
at startup and before render. Fully local, offline-native, no privacy question.

**Google Fonts CDN tier — deferred, opt-in only.** Verified: keyless catalog at
`fonts.google.com/metadata/fonts` (1,936 families; a names-only list is ~30KB raw, so
the _picker_ can ship static and offline), CSS2 endpoint live, Workbox CacheFirst on
`fonts.gstatic.com` makes a chosen font offline after first use. Tension: `base.css`
says fonts are "never a CDN", and font requests expose the user's IP to Google. If this
ships, it is an explicit per-font user action, never automatic.

## 4. Build order

1. **Layer split** ✅ (refactor, no visible change) — `vega-themes.ts` is base +
   expressive per UI theme, merged into the existing exports via `mergeChartLayers`.
2. **Preview theme selector** ✅ — global pref `ui.chartTheme`: **Astrolabe** (follows
   UI theme, default) · **Stock Vega-Lite** (empty config) · all 14 vega-themes presets ·
   (later) custom themes. Governs LivePreview **and export** (same view). Resolved
   design points: the control is a `SelectControl` in the **preview header** (a select
   nested inside PreviewSettings would close its own parent — SelectControl and
   SettingsPopover share the one-open-popover registry); Onboarding/Chart-Builder
   previews stay house-styled; preset/stock backgrounds render verbatim (a white chart
   card on the dark pane is an honest destination preview).
3. **Merge/extract config** ✅ — `core/spec-config.ts` (`mergeConfigIntoSpec`,
   `extractConfigFromSpec`), surfaced as the editor toolbar's **Config** menu
   (SelectControl action picker — council: NN/g #6, Carbon overflow; arch 10 §5 records
   the rule) with Monaco context-menu/palette as accelerators on the same functions
   (spec §03G): bake the active theme into `spec.config` (existing keys win,
   render-identical), or lift `spec.config` out to the clipboard (copy before remove —
   a failed copy aborts).
4. **Custom named themes** — new IndexedDB entity `{ name, config, fonts? }` + list UI;
   created by duplicating a preset or extract-from-spec; appears in the slice-2
   selector. Export/import as JSON alongside the library.
5. **Shipped font roster** — fontsource packages, `@font-face` registration, selector
   metadata (which themes/fonts pair), `document.fonts.load` gate in the render path,
   precache strategy above. Roster finalized via visual specimen.
6. **User font upload** — FontFace-from-IndexedDB tier; theme entity's `fonts` field
   carries `{ family, source: 'file' }`.
7. **Deferred** — Google Fonts opt-in tier; SVG export font embedding; built-in
   expressive preset gallery ("Editorial", "Terminal", "Sketch") showcasing the roster.

**Rejected:** per-snippet theme field (2026-06-12 — `spec.config` + merge/extract covers
it without a second mechanism).

## 5. Status log

- **2026-06-12 (slices 2–3)** — **theme selector + merge/extract shipped.**
  `ChartThemeId`/`chartConfigForSelection` in core; `ui.chartTheme` persisted via the
  `previewFitMode` orchestration pattern; `SelectControl` picker in the preview header;
  LivePreview renders (and therefore exports) with the selection; `core/spec-config.ts`
  merge/extract behind two Monaco editor actions. Spec updated (§03G, §04 Chart theme,
  §07, §09C) + architecture 05 §3 rewritten to the layered/selectable model. Verified:
  typecheck, eslint, full tests (802), build. Custom named themes (slice 4) and fonts
  (5–6) remain.
- **2026-06-12** — scope written; audit, vega-editor read, font research done (numbers
  above). Slice 1 (layer split) implemented.