Mosaic Declarative Specifications

The mosaic-spec package enables declarative specification of Mosaic applications as JSON or YAML files. The package provides a specification parser, as well as generators to create either running applications or JavaScript code.

For example, below is an example of a declarative specification, in either YAML or equivalent JSON, for a simple line chart. The JavaScript tab shows the code generated from the parsed specification.

Loading Example... ⏳

YAML

data:
  aapl: { file: data/stocks.parquet, where: Symbol = 'AAPL' }
plot:
- mark: lineY
  data: { from: aapl }
  x: Date
  y: Close
width: 680
height: 200

JSON

{
  "data": {
    "aapl": {
      "file": "data/stocks.parquet",
      "where": "Symbol = 'AAPL'"
    }
  },
  "plot": [
    {
      "mark": "lineY",
      "data": {
        "from": "aapl"
      },
      "x": "Date",
      "y": "Close"
    }
  ],
  "width": 680,
  "height": 200
}

JavaScript

import * as vg from "@uwdata/vgplot";

await vg.coordinator().exec([
  vg.loadParquet("aapl", "data/stocks.parquet", {where: "Symbol = 'AAPL'"})
]);

export default vg.plot(
  vg.lineY(
    vg.from("aapl"),
    {x: "Date", y: "Close"}
  ),
  vg.width(680),
  vg.height(200)
);

Using a declarative specification, we can describe an application in a standard file format, enabling portability across platforms. For example, the Mosaic Jupyter widget uses this format to pass visualization and dashboard definitions from Python to the browser.

Specification Format

Here is a slightly more complicated example, in which a dynamic Param value is added to the data before visualizing it.

Loading Example... ⏳

YAML

meta:
  title: Bias Parameter
  description: >
    Dynamically adjust queried values by adding a Param value.
    The SQL expression is re-computed in the database upon updates.
data:
  walk: { file: data/random-walk.parquet }
params:
  point: 0
vconcat:
- input: slider
  label: Bias
  as: $point
  min: 1
  max: 1000
  step: 0.1
- plot:
  - mark: areaY
    data: { from: walk }
    x: t
    y: { sql: v + $point }
    fill: steelblue
  width: 680
  height: 200

JSON

{
  "meta": {
    "title": "Bias Parameter",
    "description": "Dynamically adjust queried values by adding a Param value. The SQL expression is re-computed in the database upon updates.\n"
  },
  "data": {
    "walk": {
      "file": "data/random-walk.parquet"
    }
  },
  "params": {
    "point": 0
  },
  "vconcat": [
    {
      "input": "slider",
      "label": "Bias",
      "as": "$point",
      "min": 1,
      "max": 1000,
      "step": 0.1
    },
    {
      "plot": [
        {
          "mark": "areaY",
          "data": {
            "from": "walk"
          },
          "x": "t",
          "y": {
            "sql": "v + $point"
          },
          "fill": "steelblue"
        }
      ],
      "width": 680,
      "height": 200
    }
  ]
}

JavaScript

import * as vg from "@uwdata/vgplot";

await vg.coordinator().exec([
  vg.loadParquet("walk", "data/random-walk.parquet")
]);

const $point = vg.Param.value(0);

export default vg.vconcat(
  vg.slider({label: "Bias", as: $point, min: 1, max: 1000, step: 0.1}),
  vg.plot(
    vg.areaY(
      vg.from("walk"),
      {x: "t", y: vg.sql`v + ${$point}`, fill: "steelblue"}
    ),
    vg.width(680),
    vg.height(200)
  )
);

The example above includes descriptive metadata, data and params definitions, and nested components: an input slider and plot are positioned using a vconcat layout. The plot uses a SQL expression that includes a Param reference ($point) to define a dynamic y encoding channel.

A declarative specification may include top-level definitions for:

• meta: Metadata such as a title and description.
• config: Configuration settings, such as database extensions to load.
• data: Dataset definitions, such as files, queries, or inline data.
• params: Param & Selection definitions. All later Param references use a $ prefix.
• plotDefaults: Default plot attributes to apply to all plot instances.

The rest of the spec consists of component definitions supported by the vgplot API, including plots, inputs, and layout components.

For more, see the Specification Format Reference.

Parser & Generators Format

The parseSpec() method parses a specification in JSON format into an abstract syntax tree (AST): a structured representation of the specification content that is more convenient to analyze and manipulate.

Given a parsed AST, the generator method astToDOM() instantiates a running web application, returning Mosaic-backed Document Object Model (DOM) elements that can be added to a web page.

Meanwhile, the astToESM() method instead generates JavaScript code, in the form of an ECMAScript Module (ESM), that uses the vgplot API. The code listed in the examples above was generated using this method.

For more, see the Specification Parser & Generators Reference.