preserve dates for mean and median (#1113)

* preserve dates for mean and median

* more reducer tests

Author: mbostock

src/transforms/group.js

@@ -28,7 +28,8 @@ import {
   labelof,
   range,
   second,
-  percentile
+  percentile,
+  isTemporal
 } from "../options.js";
 import {basic} from "./basic.js";
 
@@ -260,9 +261,9 @@ export function maybeReduce(reduce, value) {
     case "max-index":
       return reduceAccessor(maxIndex);
     case "mean":
-      return reduceAccessor(mean);
+      return reduceMaybeTemporalAccessor(mean);
     case "median":
-      return reduceAccessor(median);
+      return reduceMaybeTemporalAccessor(median);
     case "variance":
       return reduceAccessor(variance);
     case "mode":
@@ -319,6 +320,15 @@ function reduceAccessor(f) {
   };
 }
 
+function reduceMaybeTemporalAccessor(f) {
+  return {
+    reduce(I, X) {
+      const x = f(I, (i) => X[i]);
+      return isTemporal(X) ? new Date(x) : x;
+    }
+  };
+}
+
 export const reduceIdentity = {
   reduce(I, X) {
     return take(X, I);

test/transforms/group-test.js

@@ -1,5 +1,6 @@
 import * as Plot from "@observablehq/plot";
 import assert from "assert";
+import * as d3 from "d3";
 
 it("Plot.group does not return unspecified options", () => {
   const A = Plot.group({});
@@ -34,3 +35,67 @@ it("Plot.group does return specified options", () => {
   assert.strictEqual("fill" in C, false);
   assert.strictEqual("stroke" in C, false);
 });
+
+it("Plot.group’s min reducer does not coerce numbers and uses natural order", () => {
+  const group = Plot.groupZ({y: "min"}, {y: "y"});
+  const data = [{y: "10"}, {y: "4"}, {y: "2"}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), ["10"]);
+});
+
+it("Plot.group’s max reducer does not coerce numbers and uses natural order", () => {
+  const group = Plot.groupZ({y: "max"}, {y: "y"});
+  const data = [{y: "10"}, {y: "4"}, {y: "2"}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), ["4"]);
+});
+
+it("Plot.group’s percentile reducer coerces numbers", () => {
+  const group = Plot.groupZ({y: "p00"}, {y: "y"});
+  const data = [{y: "1"}, {y: "4"}, {y: "2"}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), [1]);
+});
+
+it("Plot.group’s median reducer coerces numbers", () => {
+  const group = Plot.groupZ({y: "median"}, {y: "y"});
+  const data = [{y: "1"}, {y: "4"}, {y: "2"}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), [2]);
+});
+
+it("Plot.group’s median reducer ignores NaN, undefined, and null", () => {
+  const group = Plot.groupZ({y: "median"}, {y: "y"});
+  const data = [{y: "1"}, {y: "4"}, {y: null}, {y: null}, {y: NaN}, {}, {y: "2"}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), [2]);
+});
+
+it("Plot.group’s median reducer preserves dates", () => {
+  const group = Plot.groupZ({y: "median"}, {y: "y"});
+  const data = [{y: new Date("2021-01-01")}, {y: new Date("2024-01-01")}, {y: new Date("2022-01-01")}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), [new Date("2022-01-01")]);
+});
+
+it("Plot.group’s mean reducer preserves dates", () => {
+  const group = Plot.groupZ({y: "mean"}, {y: "y"});
+  const data = [{y: new Date("2021-01-01")}, {y: new Date("2024-01-01")}, {y: new Date("2022-01-01")}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), [new Date("2022-05-02T16:00:00Z")]);
+});
+
+it("Plot.group’s min reducer preserves dates", () => {
+  const group = Plot.groupZ({y: "min"}, {y: "y"});
+  const data = [{y: new Date("2021-01-01")}, {y: new Date("2024-01-01")}, {y: new Date("2022-01-01")}];
+  const facets = [d3.range(data.length)];
+  const {data: groupData} = group.transform(data, facets);
+  assert.deepStrictEqual(Plot.valueof(groupData, group.y), [new Date("2021-01-01")]);
+});