Update domain function to return all scaled-positions rather than only [min, max] (#380)

* Update domain function to return all scaled-positions rather than only [min, max]

See #376 (comment)

* Add no-args doc for scale.domain

* Add tests for multi-stop domain

Author: jo-chemla

docs/src/index.md

@@ -827,7 +827,7 @@ chroma.scale(['yellow', 'red', 'black']);
 ### scale.domain
 #### (domain)
 
-You can change the input domain to match your specific use case.
+You can change the input domain to match your specific use case. If called with no arguments, `scale.domain` returns the original array of positions along the scale where the color ramp was sampled.
 
 ```js
 // default domain is [0,1]

src/generator/scale.js

@@ -196,7 +196,7 @@ export default function (colors) {
 
     f.domain = function (domain) {
         if (!arguments.length) {
-            return _domain;
+            return _pos.map(p => _min + p * (_max - _min));;
         }
         _min = domain[0];
         _max = domain[domain.length - 1];

test/scales.test.js

@@ -410,4 +410,25 @@ describe('Some tests for scale()', () => {
             expect(f(0.625).hex()).toBe(f1(0.75).hex());
         });
     });
+
+    describe('multi-stop domain with matching ramp stops', () => {
+        const f = scale(['yellow', 'red', 'black']).domain([0, 25, 100]);
+    
+        it('returns the original domain positions', () => {
+            expect(f.domain()).toEqual([0, 25, 100]);
+        });
+    
+        it('maps first stop', () => {
+            expect(f(0).hex()).toBe('#ffff00');
+        });
+    
+        it('maps mid stop', () => {
+            expect(f(25).hex()).toBe('#ff8080');
+        });
+    
+        it('maps last stop', () => {
+            expect(f(100).hex()).toBe('#000000');
+        });
+    });
+
 });