Enables user-defined buckets in viewportHistograms

```
histogram: viewportHistogram($prop, [[0, 20], 40, 60, [80, 100]])
```

You have more control about where the values fall, making easier to
control the UI/UX by having a more stable bucket list.

Closes #1160

src/renderer/viz/expressions/aggregation/viewport/ViewportHistogram.js

@@ -7,21 +7,24 @@ import { CLUSTER_FEATURE_COUNT } from '../../../../schema';
  * Generates a histogram.
  *
  * The histogram can be based on a categorical expression, in which case each category will correspond to a histogram bar.
- * The histogram can be based on a numeric expression, in which case the minimum and maximum will be computed automatically and bars will be generated
- * at regular intervals between the minimum and maximum. The number of bars in this case is controllable through the `size` parameter.
+ *
+ * The histogram can be based on a numeric expression, the buckets for the histogram is controllable through the `sizeOrBuckets` parameter.
+ * For numeric values of sizeOrBuckets, the minimum and maximum will be computed automatically and bars will be generated at regular intervals between the minimum and maximum.
+ * When providing sizeOrBuckets as a list of buckets, the values will get assigned to the first bucket matching the criteria [bucketMin <= value < bucketMax].
  *
  * Histograms are useful to get insights and create widgets outside the scope of CARTO VL, see the following example for more info.
  *
  * @param {Number} input - expression to base the histogram
- * @param {Number} size - Optional (defaults to 20). Number of bars to use if `x` is a numeric expression
+ * @param {Number|Array} sizeOrBuckets - Optional (defaults to 20). Number of bars to use if `x` is a numeric expression; or user-defined buckets for numeric expressions.
  * @param {Number} weight - Optional. Weight each occurrence differently based on this weight, defaults to `1`, which will generate a simple, non-weighted count.
  * @return {Histogram} Histogram
  *
  * @example <caption>Create and use an histogram. (String)</caption>
  * const s = carto.expressions;
  * const viz = new carto.Viz(`
- *          \@categoryHistogram: viewportHistogram($type)
- *          \@numericHistogram:  viewportHistogram($amount, 3, 1)
+ *          \@categoryHistogram:    viewportHistogram($type)
+ *          \@numericHistogram:     viewportHistogram($amount, 3, 1)
+ *          \@userDefinedHistogram: viewportHistogram($amount, [[0, 10], [10, 20], [20, 30]], 1)
  * `);
  * ...
  * console.log(viz.variables.categoryHistogram.eval());
@@ -52,12 +55,13 @@ import { CLUSTER_FEATURE_COUNT } from '../../../../schema';
  * @api
  */
 export default class ViewportHistogram extends BaseExpression {
-    constructor (x, size = 20, weight = 1) {
+    constructor (x, sizeOrBuckets = 20, weight = 1) {
         checkMaxArguments(arguments, 3, 'viewportHistogram');
         super({ x: implicitCast(x), weight: implicitCast(weight) });
 
         this.type = 'histogram';
-        this._size = size;
+        this._sizeOrBuckets = sizeOrBuckets;
+        this._hasBuckets = Array.isArray(sizeOrBuckets);
         this._isViewport = true;
 
         this.inlineMaker = () => null;
@@ -81,7 +85,7 @@ export default class ViewportHistogram extends BaseExpression {
             }
 
             this._cached = this.x.type === 'number'
-                ? _getNumericValue(this._histogram, this._size)
+                ? (this._hasBuckets ? _getBucketsValue(this._histogram, this._sizeOrBuckets) : _getNumericValue(this._histogram, this._sizeOrBuckets))
                 : _getCategoryValue(this._histogram);
 
             return this._cached;
@@ -236,6 +240,29 @@ function _getNumericValue (histogram, size) {
     });
 }
 
+function _getBucketsValue ([...histogram], buckets) {
+    const nBuckets = buckets.length;
+    const hist = Array(nBuckets).fill(0);
+
+    for (let i = 0, len = histogram.length; i < len; i++) {
+        const x = histogram[i][0];
+        for (let j = 0; j < nBuckets; j++) {
+            const bucket = buckets[j];
+            if (x >= bucket[0] && x < bucket[1]) {
+                hist[j] += histogram[i][1];
+                break;
+            }
+        }
+    }
+
+    return hist.map((count, index) => {
+        return {
+            x: buckets[index],
+            y: count
+        };
+    });
+}
+
 function _getCategoryValue (histogram) {
     return [...histogram]
         .map(([x, y]) => {

test/unit/renderer/viz/expressions/aggregation/viewportAggregation.test.js

@@ -166,6 +166,21 @@ describe('src/renderer/viz/expressions/viewportAggregation', () => {
             ]);
         });
 
+        it('viewportHistogram($price, [[0, 1], [1, 2]], 1) should eval to the correct histogram', () => {
+            const viewportHistogram = s.viewportHistogram($price, [[0, 1.5], [1.5, 3]], 1);
+            fakeDrawMetadata(viewportHistogram);
+            expect(viewportHistogram.value).toEqual([
+                {
+                    x: [0, 1.5],
+                    y: 2
+                },
+                {
+                    x: [1.5, 3],
+                    y: 2
+                }
+            ]);
+        });
+
         it('viewportHistogram($cat) should eval to the correct histogram', () => {
             const viewportHistogram = s.viewportHistogram($cat);
             fakeDrawMetadata(viewportHistogram);