fix date typing in arrow tables

package.json

@@ -56,6 +56,7 @@
     "@types/node": "^20.5.0",
     "@typescript-eslint/eslint-plugin": "^7.2.0",
     "@typescript-eslint/parser": "^7.2.0",
+    "apache-arrow": "^16.1.0",
     "c8": "^9.1.0",
     "canvas": "^2.0.0",
     "d3-geo-projection": "^4.0.0",

src/options.js

@@ -148,6 +148,20 @@ export function arrayify(values) {
     case "Sphere":
       return [values];
   }
+
+  // Duck type Arrow tables to retype date fields to Dates. Note that we only
+  // need the first non-nullish value to be typed correctly for isTemporal to
+  // return true.
+  const fields = values?.schema?.fields;
+  if (Array.isArray(fields)) {
+    values = Array.from(values);
+    for (const f of fields) {
+      if (String(f).endsWith("<MILLISECOND>"))
+        values.some((d, i) => d[f.name] != null && (values[i] = {...values[i], [f.name]: new Date(values[i][f.name])}));
+    }
+    return values;
+  }
+
   return Array.from(values);
 }
 

test/plots/arrow-dates.ts

@@ -0,0 +1,9 @@
+import * as Plot from "@observablehq/plot";
+import * as Arrow from "apache-arrow";
+import * as d3 from "d3";
+
+export async function arrowDates() {
+  const athletes = await d3.csv<any>("data/athletes.csv", d3.autoType);
+  const table = Arrow.tableFromJSON(athletes);
+  return Plot.rectY(table, Plot.binX(undefined, {x: "date_of_birth"})).plot();
+}

test/plots/index.ts

@@ -11,6 +11,7 @@ export * from "./aapl-volume.js";
 export * from "./anscombe-quartet.js";
 export * from "./arc.js";
 export * from "./armadillo.js";
+export * from "./arrow-dates.js";
 export * from "./aspectRatio.js";
 export * from "./athletes-bins-colors.js";
 export * from "./athletes-birthdays.js";