Migrating from Vega-Lite

Prism borrows Vega-Lite’s vocabulary (mark, encoding, transform, layer, facet) and channel model. The divergences are intentional — read this guide to port specs in minutes.

At a glance

Vega-Lite	Prism	Why divergence
`data.url`	`data.source`	Pulse refs aren’t URLs (could be cohort ID, GCS path, archive#shard).
`transform[].aggregate`	same shape	identical
`op: "mean"`	same	friendly aliases match Vega-Lite verbatim
`mark`, `encoding`	same vocabulary	same
`type: "quantitative"`	same	nominal/ordinal/quantitative/temporal
`scale.scheme`	same	same color schemes
`selection`	same shape	point + interval supported v1
`params` / signals	dropped	no reactive runtime
`layer`, `concat`, `facet`, `repeat`	same	full composition v1
`condition` encodings	dropped v1	post-v1 feature
`strokeWidth` (camelCase)	`stroke_width`	snake_case throughout
Vega expression language	Pulse expression syntax	one language, no JS eval

snake_case (D019)

All field names in spec + scene IR are snake_case. Single-word Vega-Lite vocabulary (mark, encoding, transform, layer, facet) stays as-is.

Vega-Lite	Prism
`strokeWidth`	`stroke_width`
`cornerRadius`	`corner_radius`
`fontSize`	`font_size`
`tickCount`	`tick_count`
`labelOverlap`	`label_overlap`

Pulse expression syntax (D005)

filter predicates and calculate computed columns use Pulse expression syntax, not Vega’s JS-like language.

Vega-Lite	Prism
`"filter": "datum.score > 50"`	`"filter": "score > 50"`
`"filter": "datum.region === 'NA'"`	`"filter": "region == 'NA'"`
`"calculate": "datum.x * 2", "as": "y"`	`"calculate": "x * 2", "as": "y"`

No datum. prefix. == instead of ===. No JS function calls.

Aggregate aliases (D003)

Vega-Lite parity:

count sum mean median min max stdev variance q1 q3 ci0 ci1

Pulse adds: distinct mode.

Cohort-analytics extensions (Prism-only): wmean ratio lift share.

Dropped features (v1)

params / signals — no reactive runtime.
condition encodings — post-v1.
Inline Vega expressions everywhere — use Pulse expressions or pre-compute via a calculate transform.
Vega-Lite tooltip template strings — Prism tooltips are pre-formatted TooltipLine lists.

Added features

datasets block + per-layer data overrides — first-class multi-source.
Hash join transform ({join: {left, right, on, kind}, as}) — in-Prism, no Pulse change.
Cohort-analytics aggregates (wmean, lift, share, ratio).
sankey, funnel, sparkline marks — first-class, not third-party plugins.
Server-side + browser-side dataset registries.
MCP tool surface for agent integration.

Worked porting example

Vega-Lite:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {"url": "data/cars.json"},
  "transform": [{"filter": "datum.Horsepower > 100"}],
  "mark": {"type": "bar", "cornerRadius": 4},
  "encoding": {
    "x": {"field": "Origin", "type": "nominal"},
    "y": {"aggregate": "mean", "field": "Horsepower", "type": "quantitative"},
    "color": {"field": "Origin"}
  }
}

Prism:

{
  "$schema": "urn:prism:schema:v1:spec",
  "data": {"source": "cars.pulse"},
  "transform": [{"filter": "Horsepower > 100"}],
  "mark": {"type": "bar", "corner_radius": 4},
  "encoding": {
    "x": {"field": "Origin", "type": "nominal"},
    "y": {"aggregate": "mean", "field": "Horsepower", "type": "quantitative"},
    "color": {"field": "Origin", "type": "nominal"}
  }
}

Diffs:

$schema: URN form.
data.url → data.source; .json → .pulse.
filter: drop datum. prefix.
cornerRadius → corner_radius.
color channel: explicit type (Vega-Lite infers; Prism is strict).

Editor setup

prism init writes .prism/editor/ with configs for VSCode, JetBrains, Neovim, Vim — autocomplete + inline validation on *.prism.json files from the embedded JSON Schema bundle.

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