Auto: error on underspecified reduce

src/marks/auto.js

@@ -1,6 +1,6 @@
 import {ascending, InternSet} from "d3";
 import {marks} from "../mark.js";
-import {isColor, isNumeric, isObject, isOptions, isOrdinal, labelof, valueof} from "../options.js";
+import {isColor, isIterable, isNumeric, isObject, isOptions, isOrdinal, isEvery, labelof, valueof} from "../options.js";
 import {bin, binX, binY} from "../transforms/bin.js";
 import {group, groupX, groupY} from "../transforms/group.js";
 import {areaX, areaY} from "./area.js";
@@ -24,6 +24,13 @@ export function autoSpec(data, options) {
   const C = materializeValue(data, color);
   const S = materializeValue(data, size);
 
+  if (isIterable(data) && !isPrimitive(data)) {
+    for (const channel of ["x", "y", "color", "size"]) {
+      if (isUnderspecifiedReduce(options[channel]))
+        throw new Error(`setting ${channel} reducer to "${options[channel].reduce}" requires setting ${channel} field`);
+    }
+  }
+
   // Compute the default options.
   let {
     fx,
@@ -340,6 +347,17 @@ function isReducer(reduce) {
   return false;
 }
 
+function isUnderspecifiedReduce({value, reduce}) {
+  return value == null && reduce != null && !/^count$/i.test(reduce);
+}
+
+function isPrimitive(values) {
+  return isEvery(values, (d) => {
+    const type = typeof d;
+    return type === "number" || type === "boolean" || type === "string" || type === "bigint" || d instanceof Date;
+  });
+}
+
 function isHighCardinality(value) {
   return value ? new InternSet(value).size > value.length >> 1 : false;
 }

test/marks/auto-test.js

@@ -243,3 +243,42 @@ it("Plot.autoSpec makes a faceted heatmap", () => {
     colorMode: "fill"
   });
 });
+
+it("autoSpec rejects an underspecified reducer", () => {
+  // primitive array ok
+  assert.deepStrictEqual(Plot.autoSpec([74, 65], {x: Plot.identity, y: "max"}), {
+    fx: null,
+    fy: null,
+    x: {value: Plot.identity, reduce: null, zero: false},
+    y: {value: null, reduce: "max", zero: false},
+    color: {value: null, reduce: null},
+    size: {value: null, reduce: null},
+    mark: "rule",
+    markImpl: "ruleX",
+    markOptions: {
+      fx: undefined,
+      fy: undefined,
+      x: {value: Object.assign([74, 65], {label: undefined})},
+      y: undefined,
+      stroke: undefined,
+      z: undefined,
+      r: undefined,
+      tip: true
+    },
+    transformImpl: "binX",
+    transformOptions: {stroke: undefined, r: undefined, y: "max"},
+    colorMode: "stroke"
+  });
+  // in this case, reducer "max" has no input
+  assert.throws(
+    () =>
+      Plot.autoSpec(
+        [
+          {name: "Toph", height: 74},
+          {name: "Claire", height: 65}
+        ],
+        {x: "height", y: "max"}
+      ),
+    /^Error: setting y reducer to "max" requires setting y field$/
+  );
+});