adopt density.contours (#954)

* adopt density.contours

* range, not ticks

Author: mbostock

README.md

@@ -1032,7 +1032,7 @@ If a **z** channel is specified, the input points are grouped by *z*, and separa
 
 Draws contours representing the estimated density of the two-dimensional points given by the **x** and **y** channels, and possibly weighted by the **weight** channel. If either of the **x** or **y** channels are not specified, the corresponding position is controlled by the **frameAnchor** option.
 
-The **thresholds** option, which defaults to 20, specifies the approximate number of contours that will be computed at even intervals between 0 (exclusive) and the maximum density. The **bandwidth** option, which defaults to 20, specifies the standard deviation of the Gaussian kernel used for estimation in pixels.
+The **thresholds** option, which defaults to 20, specifies one more than the number of contours that will be computed at uniformly-spaced intervals between 0 (exclusive) and the maximum density (exclusive). The **bandwidth** option, which defaults to 20, specifies the standard deviation of the Gaussian kernel used for estimation in pixels.
 
 If a **z**, **stroke** or **fill** channel is specified, the input points are grouped by series, and separate sets of contours are generated for each series. If the **stroke** or **fill** is specified as *density*, a color channel is constructed with values representing the density threshold value of each contour.
 

package.json

@@ -50,7 +50,7 @@
     "vite": "2"
   },
   "dependencies": {
-    "d3": "^7.4.5",
+    "d3": "^7.5.0",
     "interval-tree-1d": "1",
     "isoformat": "0.2"
   },

src/marks/density.js

@@ -2,7 +2,7 @@ import {contourDensity, create, geoPath} from "d3";
 import {identity, maybeTuple, maybeZ, valueof} from "../options.js";
 import {Mark} from "../plot.js";
 import {coerceNumbers} from "../scales.js";
-import {applyFrameAnchor, applyDirectStyles, applyIndirectStyles, applyChannelStyles, applyTransform, distinct, groupZ} from "../style.js";
+import {applyFrameAnchor, applyDirectStyles, applyIndirectStyles, applyChannelStyles, applyTransform, groupZ} from "../style.js";
 import {initializer} from "../transforms/basic.js";
 
 const defaults = {
@@ -92,45 +92,36 @@ function densityInitializer(options, fillDensity, strokeDensity) {
         .y(Y ? i => Y[i] : cy)
         .weight(W ? i => W[i] : 1)
         .size([width, height])
-        .bandwidth(bandwidth)
-        .thresholds(thresholds);
+        .bandwidth(bandwidth);
 
-    // If there are multiple facets or multiple series, first compute the
-    // contours for each facet-series independently; choose the set of contours
-    // with the maximum threshold value (density), and then apply this set’s
-    // thresholds to all the other facet-series. TODO With API changes to
-    // d3-contour, we could avoid recomputing the blurred grid and cache
-    // individual contours, making this more efficient.
-    if (facets.length > 1 || Z && facets.length > 0 && distinct(facets[0], Z)) {
-      let maxValue = 0;
-      let maxContours = [];
-      for (const facet of facets) {
-        for (const index of Z ? groupZ(facet, Z, z) : [facet]) {
-          const C = density(index);
-          if (C.length > 0) {
-            const c = C[C.length - 1];
-            if (c.value > maxValue) {
-              maxValue = c.value;
-              maxContours = C;
-            }
-          }
-        }
+    // Compute the grid for each facet-series; find the maximum density of all
+    // grids and use this to compute contour thresholds.
+    let maxValue = 0;
+    const facetsContours = [];
+    for (const facet of facets) {
+      const facetContours = [];
+      facetsContours.push(facetContours);
+      for (const index of Z ? groupZ(facet, Z, z) : [facet]) {
+        const contour = density.contours(index);
+        const max = contour.max;
+        if (max > maxValue) maxValue = max;
+        facetContours.push([index, contour]);
       }
-      density.thresholds(maxContours.map(c => c.value));
     }
 
     // Generate contours for each facet-series.
+    const T = Array.from({length: thresholds - 1}, (_, i) => maxValue * (i + 1) / thresholds);
     const newFacets = [];
     const contours = [];
-    for (const facet of facets) {
-      const contourFacet = [];
-      newFacets.push(contourFacet);
-      for (const index of Z ? groupZ(facet, Z, z) : [facet]) {
-        for (const contour of density(index)) {
-          contourFacet.push(contours.length);
-          contours.push(contour);
-          if (FD) FD.push(contour.value * k);
-          if (SD) SD.push(contour.value * k);
+    for (const facetContours of facetsContours) {
+      const newFacet = [];
+      newFacets.push(newFacet);
+      for (const [index, contour] of facetContours) {
+        for (const t of T) {
+          newFacet.push(contours.length);
+          contours.push(contour(t));
+          if (FD) FD.push(t * k);
+          if (SD) SD.push(t * k);
           for (const key in newChannels) {
             newChannels[key].value.push(channels[key].value[index[0]]);
           }

src/style.js

@@ -184,16 +184,6 @@ function groupAesthetics({ariaLabel: AL, title: T, fill: F, fillOpacity: FO, str
   return [AL, T, F, FO, S, SO, SW, O, H].filter(c => c !== undefined);
 }
 
-export function distinct(I, X) {
-  const x = keyof(X[0]);
-  for (const i of I) {
-    if (keyof(X[i]) !== x) {
-      return true;
-    }
-  }
-  return false;
-}
-
 export function groupZ(I, Z, z) {
   const G = group(I, i => Z[i]);
   if (z === undefined && G.size > I.length >> 1) {