default identity map (#1549)

* default identity map

* identity docs

Author: mbostock

docs/transforms/map.md

@@ -132,12 +132,12 @@ Groups on the first channel of **z**, **fill**, or **stroke**, if any, and then
 Plot.mapX("cumsum", {x: d3.randomNormal()})
 ```
 
-Equivalent to Plot.map({x: *map*, x1: *map*, x2: *map*}, *options*), but ignores any of **x**, **x1**, and **x2** not present in *options*.
+Equivalent to Plot.map({x: *map*, x1: *map*, x2: *map*}, *options*), but ignores any of **x**, **x1**, and **x2** not present in *options*. In addition, if none of **x**, **x1**, or **x2** are specified, then **x** defaults to [identity](../features/transforms.md#identity).
 
 ## mapY(*map*, *options*)
 
 ```js
 Plot.mapY("cumsum", {y: d3.randomNormal()})
 ```
 
-Equivalent to Plot.map({y: *map*, y1: *map*, y2: *map*}, *options*), but ignores any of **y**, **y1**, and **y2** not present in *options*.
+Equivalent to Plot.map({y: *map*, y1: *map*, y2: *map*}, *options*), but ignores any of **y**, **y1**, and **y2** not present in *options*. In addition, if none of **y**, **y1**, or **y2** are specified, then **y** defaults to [identity](../features/transforms.md#identity).

src/transforms/map.js

@@ -1,19 +1,25 @@
 import {count, group, rank} from "d3";
-import {column, isObject, maybeInput, maybeZ, take, valueof} from "../options.js";
+import {column, identity, isObject, maybeInput, maybeZ, take, valueof} from "../options.js";
 import {basic} from "./basic.js";
 
 export function mapX(mapper, options = {}) {
-  return map(
-    Object.fromEntries(["x", "x1", "x2"].filter((key) => options[key] != null).map((key) => [key, mapper])),
-    options
-  );
+  let {x, x1, x2} = options;
+  if (x === undefined && x1 === undefined && x2 === undefined) options = {...options, x: (x = identity)};
+  const outputs = {};
+  if (x != null) outputs.x = mapper;
+  if (x1 != null) outputs.x1 = mapper;
+  if (x2 != null) outputs.x2 = mapper;
+  return map(outputs, options);
 }
 
 export function mapY(mapper, options = {}) {
-  return map(
-    Object.fromEntries(["y", "y1", "y2"].filter((key) => options[key] != null).map((key) => [key, mapper])),
-    options
-  );
+  let {y, y1, y2} = options;
+  if (y === undefined && y1 === undefined && y2 === undefined) options = {...options, y: (y = identity)};
+  const outputs = {};
+  if (y != null) outputs.y = mapper;
+  if (y1 != null) outputs.y1 = mapper;
+  if (y2 != null) outputs.y2 = mapper;
+  return map(outputs, options);
 }
 
 export function map(outputs = {}, options = {}) {

test/output/shorthandLineYWindow.svg

@@ -8,3 +8,12 @@ export async function shorthandLineY() {
   ];
   return Plot.lineY(numbers).plot();
 }
+
+export async function shorthandLineYWindow() {
+  const numbers = [
+    170.16, 172.53, 172.54, 173.44, 174.35, 174.55, 173.16, 174.59, 176.18, 177.9, 176.15, 179.37, 178.61, 177.3, 177.3,
+    177.25, 174.51, 172.0, 170.16, 165.53, 166.87, 167.17, 166.0, 159.1, 154.83, 163.09, 160.29, 157.07, 158.5, 161.95,
+    163.04, 169.79, 172.36, 172.05, 172.83, 171.8, 173.67, 176.35, 179.1, 179.26
+  ];
+  return Plot.lineY(numbers, Plot.windowY(7)).plot();
+}