Add scaleDiscretize method, tests, and documentation.

Author: jheer

README.md

@@ -81,6 +81,25 @@ Valid schemes are either arrays of color values (e.g., applicable to
 [interpolator](https://github.com/d3/d3-scale#sequential_interpolator)
 functions (e.g., applicable to `'sequential'` scales.)
 
+<a name="schemeDiscretize" href="#schemeDiscretize">#</a>
+vega.<b>schemeDiscretize</b>(<i>name</i>[, <i>schemes</i>, <i>interpolator</i>])
+[<>](https://github.com/vega/vega-scale/blob/master/src/schemes.js "Source")
+
+Registry function for adding and accessing discretized color schemes,
+consisting of an array of color schemes for specific value counts.
+The *name* argument is a String indicating the name of the color scheme.
+If the *schemes* argument is not specified, this method returns the matching
+array of color schemes value in the registry, or `null` if not found.
+If the *schemes* argument is provided, it must be an array of valid color
+arrays, with non-null entries at indices for each supported value count.
+For example, the array at index 3 should be a 3-color array. The optional
+*interpolator* argument provides a continuous color
+[interpolator](https://github.com/d3/d3-scale#sequential_interpolator)
+to use when a specific item count is not provided or undefined. If the
+*interpolator* argument is not provided, an interpolator will be
+automatically created using basis spline interpolation in the RGB color
+space for the last (largest) color array in *schemes*.
+
 <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")

index.js

@@ -1,5 +1,5 @@
-export {default as scale} from './src/scales';
-export {default as scheme} from './src/schemes';
 export {default as bandSpace} from './src/bandSpace';
+export {default as scale} from './src/scales';
 export {interpolate, interpolateRange, scaleFraction} from './src/interpolate';
+export {scheme, schemeDiscretized} from './src/schemes';
 export {timeInterval, utcInterval} from './src/timeInterval';

src/schemes.js

@@ -7,7 +7,7 @@ import * as _ from 'd3-scale-chromatic';
 import {interpolateRgbBasis} from 'd3-interpolate';
 import {peek} from 'vega-util';
 
-var discrete = {
+var discretized = {
   blueorange:  blueOrange
 };
 
@@ -46,7 +46,7 @@ var schemes = {
 
 function add(name, suffix) {
   schemes[name] = _['interpolate' + suffix];
-  discrete[name] = _['scheme' + suffix];
+  discretized[name] = _['scheme' + suffix];
 }
 
 // sequential single-hue
@@ -82,7 +82,7 @@ add('yellowgreen',       'YlGn');
 add('yelloworangebrown', 'YlOrBr');
 add('yelloworangered',   'YlOrRd');
 
-export default function(name, scheme) {
+export function scheme(name, scheme) {
   if (arguments.length > 1) {
     schemes[name] = scheme;
     return this;
@@ -92,7 +92,19 @@ export default function(name, scheme) {
   name = part[0];
   part = +part[1] + 1;
 
-  return part && discrete.hasOwnProperty(name) ? discrete[name][part-1]
+  return part && discretized.hasOwnProperty(name) ? discretized[name][part-1]
     : !part && schemes.hasOwnProperty(name) ? schemes[name]
     : undefined;
 }
+
+export function schemeDiscretized(name, schemeArray, interpolator) {
+  if (arguments.length > 1) {
+    discretized[name] = schemeArray;
+    schemes[name] = interpolator || interpolateRgbBasis(peek(schemeArray));
+    return this;
+  }
+
+  return discretized.hasOwnProperty(name)
+    ? discretized[name]
+    : undefined;
+}

test/scheme-test.js

@@ -0,0 +1,27 @@
+var tape = require('tape'),
+    vega = require('../');
+
+tape('scheme registers a single color scheme', function(test) {
+  var name = 'rgb',
+      colors = ['#f00', '#0f0', '#00f'];
+
+  test.equal(vega.scheme(name), undefined);
+  vega.scheme(name, colors);
+  test.deepEqual(vega.scheme(name), colors);
+  test.end();
+});
+
+tape('schemeDiscretized registers discretized color schemes', function(test) {
+  var name = 'rgbd',
+      colors = [
+        ['#f00'],
+        ['#f00', '#0f0'],
+        ['#f00', '#0f0', '#00f']
+      ];
+
+  test.equal(vega.schemeDiscretized(name), undefined);
+  vega.schemeDiscretized(name, colors);
+  test.deepEqual(vega.schemeDiscretized(name), colors);
+  test.equal(typeof vega.scheme(name), 'function');
+  test.end();
+});