expose identity reducer (#1382)

* expose identity reducer

* test identity

* document: "identity" reducer support for bin, group and hexbin
test hexbin

---------

Co-authored-by: Philippe Rivière <[email protected]>

Author: mbostock

README.md

@@ -2121,6 +2121,7 @@ The following aggregation methods are supported:
 * *pXX* - the percentile value, where XX is a number in [00,99]
 * *deviation* - the standard deviation
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
+* *identity* - the array of values
 * *x* - the middle of the bin’s *x* extent (when binning on *x*)
 * *x1* - the lower bound of the bin’s *x* extent (when binning on *x*)
 * *x2* - the upper bound of the bin’s *x* extent (when binning on *x*)
@@ -2278,6 +2279,7 @@ The following aggregation methods are supported:
 * *pXX* - the percentile value, where XX is a number in [00,99]
 * *deviation* - the standard deviation
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
+* *identity* - the array of values
 * a function - passed the array of values for each group
 * an object with a *reduceIndex* method, an optionally a *scope*
 
@@ -2852,6 +2854,7 @@ The following aggregation methods are supported:
 * *deviation* - the standard deviation
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
 * *mode* - the value with the most occurrences
+* *identity* - the array of values
 * a function to be passed the array of values for each bin and the extent of the bin
 * an object with a *reduceIndex* method
 

src/reducer.d.ts

@@ -27,12 +27,14 @@ export type ReducerPercentile =
  * - *variance* - the variance per [Welford’s algorithm][1]
  * - *mode* - the value with the most occurrences
  * - *pXX* - the percentile value, where XX is a number in [00,99]
+ * - *identity* - the array of values
  *
  * [1]: https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm
  */
 export type ReducerName =
   | "first"
   | "last"
+  | "identity"
   | "count"
   | "distinct"
   | "sum"

src/transforms/group.js

@@ -245,6 +245,8 @@ export function maybeReduce(reduce, value, fallback = invalidReduce) {
       return reduceFirst;
     case "last":
       return reduceLast;
+    case "identity":
+      return reduceIdentity;
     case "count":
       return reduceCount;
     case "distinct":

test/output/gridReduceIdentity.svg

@@ -36,6 +36,62 @@ export async function gridChoropleth() {
   });
 }
 
+export async function gridChoroplethDx() {
+  const [grid, data] = await Promise.all([
+    await d3.csv<any>("data/us-state-grid.csv", d3.autoType).then(gridmap),
+    await d3.csv<any>("data/us-state-population-2010-2019.csv", d3.autoType)
+  ]);
+  const states = data.filter((d) => grid.has(d.State)).map((d) => ({...d, ...grid.get(d.State)}));
+  return Plot.plot({
+    height: 420,
+    x: {
+      axis: null
+    },
+    y: {
+      axis: null
+    },
+    color: {
+      type: "diverging-log",
+      scheme: "piyg"
+    },
+    marks: [
+      Plot.cell(states, {x: "x", y: "y", fill: change, dx: -1.5, dy: -1.5}),
+      Plot.cell(states, {x: "x", y: "y", stroke: "black", dx: 1.5, dy: 1.5}),
+      Plot.text(states, {x: "x", y: "y", text: "key", dy: -6}),
+      Plot.text(states, {
+        x: "x",
+        y: "y",
+        text: (
+          (f) => (d) =>
+            f(change(d) - 1)
+        )(d3.format("+.0%")),
+        dy: 6,
+        fillOpacity: 0.6
+      })
+    ]
+  });
+}
+
+export async function gridReduceIdentity() {
+  const grid = await d3.csv<any>("data/us-state-grid.csv", d3.autoType);
+  return Plot.plot({
+    axis: null,
+    x: {insetRight: 200},
+    y: {reverse: true},
+    aspectRatio: true,
+    marks: [
+      Plot.rect(grid, {x: "x", y: "y", stroke: "currentColor", interval: 1, inset: 3}),
+      Plot.text(
+        grid,
+        Plot.groupY(
+          {y: ([y]) => y + 0.5, text: "identity"},
+          {sort: "x", frameAnchor: "right", y: "y", text: "key", dx: -10, stroke: "white", fill: "currentColor"}
+        )
+      )
+    ]
+  });
+}
+
 function gridmap(states: {name: string}[]) {
   return new Map(states.map((state) => [state.name, state]));
 }

test/output/hexbinIdentityReduce.svg

@@ -15,3 +15,22 @@ export async function hexbinZ() {
     ]
   });
 }
+
+export async function hexbinIdentityReduce() {
+  const penguins = await d3.csv<any>("data/penguins.csv", d3.autoType);
+  return Plot.plot({
+    inset: 10,
+    height: 600,
+    marks: [
+      Plot.hexgrid({binWidth: 8, strokeOpacity: 0.05}),
+      Plot.frame(),
+      Plot.text(
+        penguins,
+        Plot.hexbin(
+          {text: "identity"},
+          {x: "culmen_length_mm", y: "body_mass_g", text: (d) => d.species[0], binWidth: 8, fontSize: 7}
+        )
+      )
+    ]
+  });
+}

test/plots/grid-choropleth-dx.ts

@@ -97,7 +97,6 @@ export * from "./gistemp-anomaly.js";
 export * from "./google-trends-ridgeline.js";
 export * from "./graticule.js";
 export * from "./greek-gods.js";
-export * from "./grid-choropleth-dx.js";
 export * from "./grid-choropleth.js";
 export * from "./grouped-rects.js";
 export * from "./hadcrut-warming-stripes.js";