apply maybeReduce to data channel

Author: mbostock

src/transforms/bin.js

@@ -2,7 +2,7 @@ import {bin as binner} from "d3";
 import {firstof} from "../defined.js";
 import {valueof, range, identity, maybeLazyChannel, maybeTransform, maybeTuple, maybeColor, maybeValue, mid, labelof} from "../mark.js";
 import {offset} from "../style.js";
-import {maybeGroup, maybeGroupOutputs, reduceIdentity} from "./group.js";
+import {maybeGroup, maybeGroupOutputs, maybeReduce, reduceIdentity} from "./group.js";
 
 // Group on {z, fill, stroke}, then optionally on y, then bin x.
 export function binX(outputs, {inset, insetLeft, insetRight, ...options} = {}) {
@@ -61,6 +61,7 @@ function binn(
 ) {
   bx = maybeBin(bx);
   by = maybeBin(by);
+  reduceData = maybeReduce(reduceData, identity);
   outputs = maybeGroupOutputs(outputs, inputs);
 
   // Produce x1, x2, y1, and y2 output channels as appropriate (when binning).

src/transforms/group.js

@@ -33,9 +33,10 @@ export function group(outputs, options = {}) {
 function groupn(
   x, // optionally group on x
   y, // optionally group on y
-  {data: reduceData = reduceIdentity, ...outputs} = {}, // output channel definitions
+  {data: reduceData = reduceFirst, ...outputs} = {}, // output channel definitions
   inputs = {} // input channels and options
 ) {
+  reduceData = maybeReduce(reduceData, identity);
   outputs = maybeGroupOutputs(outputs, inputs);
 
   // Produce x and y output channels as appropriate.
@@ -133,7 +134,7 @@ export function maybeGroup(I, X) {
   return X ? sort(grouper(I, i => X[i]), first) : [[, I]];
 }
 
-function maybeReduce(reduce, value) {
+export function maybeReduce(reduce, value) {
   if (reduce && typeof reduce.reduce === "function") return reduce;
   if (typeof reduce === "function") return reduceFunction(reduce);
   switch ((reduce + "").toLowerCase()) {