fix crash during reindex (#2028)

src/options.js

@@ -25,7 +25,7 @@ export function valueof(data, value, type) {
 }
 
 function maybeTake(values, index) {
-  return index ? take(values, index) : values;
+  return values != null && index ? take(values, index) : values;
 }
 
 function maybeTypedMap(data, f, type) {

test/plots/index.ts

@@ -164,6 +164,7 @@ export * from "./metro-unemployment-ridgeline.js";
 export * from "./metro-unemployment-slope.js";
 export * from "./metro-unemployment-stroke.js";
 export * from "./metro-unemployment.js";
+export * from "./mixed-facets.js";
 export * from "./moby-dick-faceted.js";
 export * from "./moby-dick-letter-frequency.js";
 export * from "./moby-dick-letter-pairs.js";

test/plots/mixed-facets.ts

@@ -0,0 +1,20 @@
+import * as Plot from "@observablehq/plot";
+
+export async function mixedFacets() {
+  const data = [
+    {date: new Date("2024-01-01"), name: "a", value: 1},
+    {date: new Date("2024-01-01"), name: "b", value: 2},
+    {date: new Date("2024-01-01"), name: "a", value: 2},
+    {date: new Date("2024-02-01"), name: "b", value: 3},
+    {date: new Date("2024-02-01"), name: "a", value: 5},
+    {date: new Date("2024-02-01"), name: "b", value: 2},
+    {date: new Date("2024-02-01"), name: "a", value: 3}
+  ];
+  return Plot.plot({
+    marks: [
+      Plot.barY(data, {x: "name", y: "value", fill: "name", fx: "date", fy: "name"}),
+      Plot.barY(data, {x: "name", y: "value", fx: "date", stroke: "currentColor"}),
+      Plot.ruleY([0])
+    ]
+  });
+}