# vega-scale

Scales and color schemes for visual encoding.

This module provides [scale](#scale) and [scheme](#scheme) methods for
managing scale mappings and color schemes. By default, the scale and
scheme registries include all scale types and color schemes provided
by the D3 4.0 [d3-scale](https://github.com/d3/d3-scale) and
[d3-scale-chromatic](https://github.com/d3/d3-scale-chromatic) modules.

This module also provides augmented implementations of `'band'`, `'point'`,
and `'sequential'` scales in order to provide improved layout and
inversion support for band/point scales, and multi-domain and color range
array support for sequential scales.

## API Reference

<a name="scale" href="#scale">#</a>
vega.<b>scale</b>(<i>type</i>[, <i>scale</i>])
[<>](https://github.com/vega/vega-scale/blob/master/src/scales.js "Source")

Registry function for adding and accessing scale constructor functions.
The *type* argument is a String indicating the name of the scale type.
If the *scale* argument is not specified, this method returns the matching
scale constructor in the registry, or `null` if not found.
If the *scale* argument is provided, it must be a scale constructor function
to add to the registry under the given *type* name.

By default, the scale registry includes entries for all scale types provided
by D3 4.0's [d3-scale](https://github.com/d3/d3-scale) module. Scales created
using the constructor returned by this method have an additional `type`
property indicating the scale type. All scales supporting either an `invert`
or `invertExtent` method are augmented with an additional `invertRange`
function that returns an array of corresponding domain values for a given
interval in the scale's output range.

```js
// linear scale
var linear = vega.scale('linear');
var scale = linear().domain([0, 10]).range([0, 100]);
scale.type; // 'linear'
scale.invertRange([0, 100]); // [0, 10]
```

```js
var ordinal = vega.scale('ordinal');

// ordinal scale
var scale1 = ordinal().domain(['a', 'b', 'c']).range([0, 1, 2]);
scale1.type; // 'ordinal'

// ordinal scale with range set to the 'category20' color palette
var scale2 = ordinal().range(vega.scheme('category20'));
```

```js
var seq = vega.scale('sequential');

// sequential scale, using the plasma color palette
var scale1 = seq().interpolator(vega.scheme('plasma'));
scale1.type; // 'sequential'
```

<a name="scheme" href="#scheme">#</a>
vega.<b>scheme</b>(<i>name</i>[, <i>scheme</i>])
[<>](https://github.com/vega/vega-scale/blob/master/src/schemes.js "Source")

Registry function for adding and accessing color schemes.
The *name* argument is a String indicating the name of the color scheme.
If the *scheme* argument is not specified, this method returns the matching
scheme value in the registry, or `null` if not found.
If the *scheme* argument is provided, it must be a valid color array or
[interpolator](https://github.com/d3/d3-scale#sequential_interpolator)
to add to the registry under the given *name*.

By default, the scheme registry includes entries for all scheme types
provided by D3 4.0's [d3-scale](https://github.com/d3/d3-scale) and
[d3-scale-chromatic](https://github.com/d3/d3-scale-chromatic) module.
Valid schemes are either arrays of color values (e.g., applicable to
`'ordinal'` scales) or
[interpolator](https://github.com/d3/d3-scale#sequential_interpolator)
functions (e.g., applicable to `'sequential'` scales.)

<a name="interpolate" href="#interpolate">#</a>
vega.<b>interpolate</b>(<i>name</i>[, <i>gamma</i>])
[<>](https://github.com/vega/vega-scale/blob/master/src/interpolate.js "Source")

Returns the D3 interpolator factory with the given *name* and optional
*gamma*. All interpolator types provided by the
[d3-interpolate](https://github.com/d3/d3-interpolate) module are supported.
However, Vega uses hyphenated rather than camelCase names.

```js
var rgbBasis = vega.interpolate('rgb-basis'); // d3.interpolateRgbBasis
var rgbGamma = vega.interpolate('rgb', 2.2);  // d3.interpolateRgb.gamma(2.2)
```

<a name="interpolateRange" href="#interpolateRange">#</a>
vega.<b>interpolateRange</b>(<i>interpolator</i>, <i>range</i>])
[<>](https://github.com/vega/vega-scale/blob/master/src/interpolate.js "Source")

Given a D3 *interpolator* instance, return a new interpolator with a modified
interpolation *range*. The *range* argument should be a two element array
whose entries lie in the range [0, 1]. This method is convenient for
transforming the range of values over which interpolation is performed.

```js
var number = d3.interpolateNumber(0, 10);
number(0);   // 0
number(0.5); // 5
number(1);   // 1

var range = vega.interpolateRange(number, [0.2, 0.8]);
range(0);   // 2
range(0.5); // 5
range(1);   // 8
```

<a name="timeInterval" href="#timeInterval">#</a>
vega.<b>timeInterval</b>(<i>unit</i>)
[<>](https://github.com/vega/vega-scale/blob/master/src/timeInterval.js "Source")

Given a string _unit_, return a corresponding
[D3 time interval](https://github.com/d3/d3-time#_interval) function.
Valid _unit_ strings are: `"millisecond"`, `"second"`, `"minute"`, `"hour"`,
`"day"`, `"week"`, `"month"`, and `"year"`.

<a name="utcInterval" href="#utcInterval">#</a>
vega.<b>utcInterval</b>(<i>unit</i>)
[<>](https://github.com/vega/vega-scale/blob/master/src/timeInterval.js "Source")

Given a string _unit_, return a corresponding UTC-variant of a
[D3 time interval](https://github.com/d3/d3-time#_interval) function.
Valid _unit_ strings are: `"millisecond"`, `"second"`, `"minute"`, `"hour"`,
`"day"`, `"week"`, `"month"`, and `"year"`.