Parallel Coordinates

Cars of 1970 connected across fuel economy, horsepower, weight, and acceleration axes — One source row connected across four independently scaled dimensions.

Use Parallel coordinates when each row should remain visible while several measurements are compared. Every eligible source row becomes one selectable open path. Every dimension owns a local scale and ordinary line/text axis.

Complete chart action

createParallelCoordinates({
  id?, data?, coordinate?, dimensions, key?, missing?,
  color?, strokeDash?, line?, guides?
}): ChartProgram

Only dimensions is required after a Canvas and an unambiguous dataset exist.

import { chart } from "ggaction";

const program = chart()
  .createCanvas({
    width: 860,
    height: 500,
    margin: { top: 110, right: 160, bottom: 65, left: 78 }
  })
  .createData({ values: cars })
  .filterData({
    id: "cars1970",
    field: "Year",
    oneOf: ["1970-01-01"]
  })
  .createParallelCoordinates({
    dimensions: [
      { field: "Miles_per_Gallon", title: "MPG", scale: { nice: true, zero: false } },
      { field: "Horsepower", scale: { nice: true, zero: false } },
      { field: "Weight_in_lbs", title: "Weight (lb)", scale: { nice: true, zero: false } },
      { field: "Acceleration", scale: { nice: true, zero: false } }
    ],
    key: "Name",
    color: {
      field: "Origin",
      fieldType: "nominal",
      scale: { palette: "tableau10" }
    },
    line: { strokeWidth: 1.25, opacity: 0.48 }
  });

The stable mark ID is parallelCoordinates; the coordinate ID is parallel. Pass explicit IDs only when those roles are already occupied or several compatible resources make inference ambiguous.

Dimensions

A dimension is a field string or an object:

type ParallelDimension = string | {
  field: string;
  fieldType?: "quantitative" | "ordinal";
  title?: string;
  scale?: Omit<ScaleOptions, "id">;
};

At least two unique fields are required, and array order is axis order.
Finite numeric values infer quantitative; consistent strings infer ordinal. Declare numeric categories as ordinal explicitly.
title defaults to the field name.
Scale IDs are namespaced from the mark and dimension position. Supply scale behavior such as type, domain, range, nice, zero, or reverse, not another id.

Row identity and missing values

key names a dataset field whose values must be present and unique. It is optional: omission uses stable source-row lineage and never guesses a field.

`missing`	Result
`"break"`	Default. Keep the row item and draw each valid multi-point fragment.
`"drop-row"`	Remove any row with an invalid dimension value.
`"error"`	Reject the action before changing the program.

Advanced encoding

Use the atomic encoding after creating a line mark when the wrapped steps need to remain individually controllable:

const advanced = chart()
  .createCanvas()
  .createData({ values })
  .createLineMark({ id: "profiles" })
  .encodeParallelCoordinates({
    target: "profiles",
    dimensions: ["first", "second", "third"]
  })
  .createGuides();

encodeParallelCoordinates replaces the complete ordered assignment atomically. It cannot mix with x/y or theta/radius position encodings on the same layer.

Appearance, guides, and revisions

color and strokeDash reuse the line-series encoding vocabulary.
line accepts open linear-line appearance: stroke, strokeWidth, and opacity. Curved and closed paths are rejected.
Omitted guides create dimension axes and any applicable legend. Use guides: false to omit all guides.
editCanvas, dimension editScale, data revisions, and filterMarks rematerialize paths, axes, and legends together.
selectMarks, highlightMarks, and filterMarks operate at source-row grain. Text is not attached automatically because a path has no unique text anchor.

Limitations

Temporal dimensions, curved paths, path bundling, interactive brushing, drag-to-reorder axes, faceting, and shared Parallel axes across composed child programs are not implemented.

Basic Charts · Coordinates · Series encodings · Selection and Highlighting