fixes two issues with temporal contours:

1. blur2 does not support dates
2. the contour ticks should be derived as utc ticks not numerical ticks

closes #2250

src/marks/contour.js

@@ -1,7 +1,7 @@
-import {blur2, contours, geoPath, max, min, nice, range, ticks, thresholdSturges} from "d3";
+import {blur2, contours, geoPath, max, min, nice, range, ticks, thresholdSturges, scaleUtc} from "d3";
 import {createChannels} from "../channel.js";
 import {create} from "../context.js";
-import {labelof, identity, arrayify, map} from "../options.js";
+import {labelof, identity, arrayify, map, isTemporal} from "../options.js";
 import {applyPosition} from "../projection.js";
 import {applyChannelStyles, applyDirectStyles, applyIndirectStyles, applyTransform, styles} from "../style.js";
 import {initializer} from "../transforms/basic.js";
@@ -115,7 +115,8 @@ function contourGeometry({thresholds, interval, ...options}) {
     const {pixelSize: k, width: w = Math.round(Math.abs(dx) / k), height: h = Math.round(Math.abs(dy) / k)} = this;
     const kx = w / dx;
     const ky = h / dy;
-    const V = channels.value.value;
+    const temporal = isTemporal(channels.value.value);
+    const V = temporal && this.blur > 0 ? Float64Array.from(channels.value.value) : channels.value.value;
     const VV = []; // V per facet
 
     // Interpolate the raster grid, as needed.
@@ -149,7 +150,7 @@ function contourGeometry({thresholds, interval, ...options}) {
     if (this.blur > 0) for (const V of VV) blur2({data: V, width: w, height: h}, this.blur);
 
     // Compute the contour thresholds.
-    const T = maybeTicks(thresholds, V, ...finiteExtent(VV));
+    const T = maybeTicks(thresholds, V, ...finiteExtent(VV), temporal);
     if (T === null) throw new Error(`unsupported thresholds: ${thresholds}`);
 
     // Compute the (maybe faceted) contours.
@@ -187,10 +188,11 @@ function contourGeometry({thresholds, interval, ...options}) {
 // thresholds across facets. When an interval is used, note that the lowest
 // threshold should be below (or equal) to the lowest value, or else some data
 // will be missing.
-function maybeTicks(thresholds, V, min, max) {
+function maybeTicks(thresholds, V, min, max, temporal) {
   if (typeof thresholds?.range === "function") return thresholds.range(thresholds.floor(min), max);
   if (typeof thresholds === "function") thresholds = thresholds(V, min, max);
   if (typeof thresholds !== "number") return arrayify(thresholds);
+  if (temporal) return scaleUtc().domain([min, max]).nice(thresholds).ticks(thresholds);
   const tz = ticks(...nice(min, max, thresholds), thresholds);
   while (tz[tz.length - 1] >= max) tz.pop();
   while (tz[1] < min) tz.shift();

test/plots/walmarts.ts

@@ -1,6 +1,6 @@
 import * as Plot from "@observablehq/plot";
 import * as d3 from "d3";
-import {mesh} from "topojson-client";
+import {feature, mesh} from "topojson-client";
 
 export async function walmarts() {
   const [walmarts, statemesh] = await Promise.all([
@@ -36,3 +36,34 @@ export async function walmarts() {
     ]
   });
 }
+
+export async function walmartsDateContours() {
+  const [walmarts, [statemesh, nation]] = await Promise.all([
+    d3.tsv<any>("data/walmarts.tsv", d3.autoType),
+    d3.json<any>("data/us-counties-10m.json").then((us) => [
+      mesh(us, {
+        type: "GeometryCollection",
+        geometries: us.objects.states.geometries.filter((d) => d.id !== "02" && d.id !== "15")
+      }),
+      feature(us, us.objects.nation)
+    ])
+  ]);
+  return Plot.plot({
+    width: 960,
+    height: 600,
+    projection: "albers",
+    color: {legend: true, label: "Mean opening date"},
+    clip: nation,
+    marks: [
+      Plot.contour(walmarts, {
+        x: "longitude",
+        y: "latitude",
+        fill: "date",
+        interpolate: "random-walk",
+        blur: 5
+      }),
+      Plot.geo(statemesh, {strokeOpacity: 0.25}),
+      Plot.geo(nation, {strokeWidth: 1.5})
+    ]
+  });
+}