Candela JavaScript API


Candela comes with several visualization components ready to use. To make it easier to include these components in your project, they are partitioned into several built-in plugins. Each plugin exports its contents through its index.js file:

import * as candelaVega from 'candela/plugins/vega';

let vis = new candelaVega.BarChart(...);

and can load its contents into the candela.components object through its load.js file as follows:

import candela from 'candela';
import 'candela/plugins/vega/load.js';

let vis = new candela.components.BarChart(...);

You can also import a component directly:

import BarChart from 'candela/plugins/vega/BarChart';

let vis = new BarChart(...);

And as a last resort, you can also import the candela bundle, which is built to contain every component, preloaded into candela.components:

import candela from 'candela/dist/candela';

let vis = new candela.components.BarChart(...);

However, the candela bundle is very large; using one of the other methods of building your application will result in a smaller, more manageable bundle size.

The current list of plugins is:

  • candela/plugins/vega - Charts based on Vega, including basic chart types such as bar charts, scatter plots, and histograms.
  • candela/plugins/geojs - Components based on GeoJS for geospatial data visualization.
  • candela/plugins/glo - A component based on GLO - “graph-level operations”.
  • candela/plugins/lineup - A component based on LineUp for visualizing rankings.
  • candela/plugins/onset - A component based on OnSet for visualizing subset relationships.
  • candela/plugins/sententree - A component based on SentenTree for visualizing the grammatical structure of a corpus of text.
  • candela/plugins/similaritygraph - A specialized interactive graph visualization component for investigating degrees of similarity between nodes in a data table.
  • candela/plugins/trackerdash - A component based on the TrackerDash algorithm metric tracking dashboard.
  • candela/plugins/treeheatmap - A heatmap combined with hierarchical clustering.
  • candela/plugins/upset - A component based on UpSet, also for visualizing subset relationships.

For more details about each component (including how to import these bundles into your project), see the full list of component documentation.


Components often have a width and height option, which specify the width and height of the component in pixels.

Field matchings

Axis scales

Several components have options that relate to the axes of the visualization. These are commonly called x and y but may also have more descriptive names. The component will often automatically detect the type of values in the field being mapped to an axis and will create an appropriate axis type, such as evenly-spaced values for string fields and a continuous-ranged axis for numeric and date fields. Visualizations showing continuous-ranged axes often allow pan and zoom of the axis by dragging and scrolling in the visualization area.

Color scales

Many Candela components contain a color option, which will color the visual elements by the field specified. When possible, color will detect the type of the column and use an appropriate color scale. For fields containing string/text values, the visualization will use a color scale with distinct colors for each unique value. For fields containing numeric or date values, the visualization will use a smooth color gradient from low to high values.

Data types


A Candela table is an array of records of the form:

    "a": 1,
    "b": "Mark",
    "c": "Jun 1, 2010"
    "a": 2,
    "b": "Andy",
    "c": "Feb 6, 2010"
    "a": 3,
    "b": "Joe",
    "c": "Nov 27, 2010"

Visualization components

VisComponent is the base class for Candela visualization components. This class is intentionally minimal, because there are only a few common features of all Candela components:

  1. Candela components work on the web, so the constructor looks like new VisComponent(el), where el is (usually) a DOM element. The VisComponent constructor attaches el to the object, so you can always refer to it using this.el.
  2. Candela components perform some type of visualization, so they have a render method. The base class render simply raises an exception.
  3. Sometimes you need to change an aspect of the visualization at runtime, such as the color map, which columns of data are being visualized, or even the data itself; to support such changes, Candela components have an update method. The base class update returns a promise object that delivers the component itself.
  4. When a visualization component reaches the end of its lifecycle, it may need to clean up after itself, which can be done in the component’s destroy method. The base class destroy simply removes all content from this.el.

You can create a concrete visualization component by extending VisComponent. The following best practices maximize clarity, reusability, and interoperability of your components (in the rest of this document, imagine that Component is declared as an extension of VisComponent, such as BarChart):

  1. The constructor should take an additional parameter options encapsulating all runtime options for the component.
  2. The component should report its expected inputs in Component.options.
var component = new Component(el, options)

Constructs a new instance of the Candela component.

  • el is a valid container for the visualization. The container will often be a DOM element such as <div>, but may need to be another type for certain visualizations.
  • options is an object containing the initial options for the visualization. This includes any data, visual matchings, or other settings pertinent to the visualization. Options are specified in the form {name: value}.

Note: The constructor for the abstract superclass is empty. You should use the constructor for specific subclasses of VisComponent.


Renders the component to its container using the current set of options.

Note: The VisComponent render() method simply throws an exception; if you truly want your component to do nothing when it renders, simply redefine the method to be a no-op.


Changes the component state to reflect options. This method allows for incremental changes to the component state. The form of options should be the same as what the constructor takes. The difference is, only the options given to this method should change, while any left unspecified should remain as they are.

Note: The VisComponent update() method returns a promise object that delivers the component itself without changing it, since the semantics of updating will be different for every component.


Performs any cleanup required of the component when it is no longer needed. This may be as simple as emptying the container element the component has been using, or it may involve unregistering event listeners, etc.

Note: The VisComponent destroy() method just empties the top-level container, since this is a common “cleanup” operation.


Convenience method that empties the component’s container element. This can be used in the constructor to prepare the container element, or in the destroy method to clean up after the component.


The serializationFormats field is a list of strings of supported formats. Formats include:

  • 'png': A base64-encoded string for a PNG image. The string may be placed in the src attribute of an <img> element to show the image.
  • 'svg': A base64-encoded string for an SVG scene. The string may be placed in the src attribute of an <img> element to show the image.

Serializes the component into the specified format.


This static property is an array of Option specifications, containing a description of the options this visualization accepts. This may be used to introspect the component to implement features such as automatic UI building.


A static field containing the type of container this visualization can be added to. The most common is DOMElement.


Candela uses mixins to add functionality to VisComponent when creating a new component. To use a mixin, the pattern is as follows:

class MyCoolVisualization extends Mixin(candela.VisComponent) {

The class Mixin is defined using this pattern:

const Mixin = Base => class extends Base {
  mixinMethod() {

This is a function expression that maps a base class to a new, unnamed class - in other words, mixins are functions that can be applied to VisComponent (or any existing component class) to yield a new class with extra functionality.

Candela comes with several mixins, which are available in the plugin candela/plugins/mixin.


Adds basic event handling to the component. The component gains an .on() method that takes a string naming an event type, and a callback to invoke when that event occurs, and a .trigger() method that takes an event type and optional arguments to fire that event type with those arguments.


Causes a width and height property to be written to the component, based on the size of this.el at the time the component is instantiated.


Uses the Events mixin to trigger a resize event whenever the containing element’s size changes. The event fires with the new width and height of the element, and a reference to the component itself.


Combines the InitSize and Resize mixins, and automatically responds to the resize event by updating the this.width and this.height properties.


Implements a Vega or Vega-Lite visualization component. Subclasses should implement a generateSpec() method which returns an appropriate Vega/Vega-Lite specification based on the view options.

Option specification

An option specification describes an input to a visualization as part of the Component.options array. It is an object containing the following properties:

name (String)
The name of the option.
type (String)
The type of the option. Type and format follow Girder Worker types/formats.
format (String)
The format (specific encoding within a type) of the option. Type and format follow Girder Worker types/formats.
domain (Domain)
Optional. A restriction on this option’s set of allowed values.

Domain specification

The domain of an option restricts the set of allowed values for an option. It is an object with the following properties:

mode (String)
The domain mode, one of 'choice' or 'field'. The 'choice' mode will allow a fixed set of options set in the 'from' field. The 'field' mode will allow a field or list of fields from another input. If the option type is 'string', the option is a single field, and if the option type is 'string_list', the option accepts a list of fields.
from (Array or String)
If the mode is 'choice', it is an array of strings to use as a dropdown. If the mode is 'field', it is the name of the input from which to extract fields.
fieldTypes (Array)
If mode is 'field', this specifies the types of fields to support. This array may contain any combination of datalib’s supported field types which include 'string', 'date', 'number', 'integer', and 'boolean'.


Candela utility functions.


Returns an object with the fields width and height containing the current width and height of the DOM element el in pixels.

Utilities for generating Vega specifications.
util.vega.chart(template, el, options, done)

Generates a Vega chart based on a template instantiated with options.

template is the [Vega template](#vega-templates) representing the chart.

el is the DOM element in which to place the Vega visualization.

options is an object of {key: value} pairs, containing the options to use while compiling the template. The options may contain arbitrarily nested objects and arrays.

done is a callback function to called when the Vega chart is generated. The function takes one argument that is the resulting Vega chart.

util.vega.transform(template, options)

Returns the instantiation of a template with the given options. This is the underlying function used by :js:func`util.vega.chart` to instantiate its template before rendering with the Vega library.

template is the Vega template.

options is an object of {key: value} pairs, containing the options to use while compiling the template. The options may contain arbitrarily nested objects and arrays.