better error message for missing stack value (#1798)

* better error message for missing channel

* linked pointer example

* shorten error message

src/transforms/stack.js

@@ -76,6 +76,7 @@ function mergeOptions(options) {
 const lengthy = {length: true};
 
 function stack(x, y = one, kx, ky, {offset, order, reverse}, options) {
+  if (y === null) throw new Error(`stack requires ${ky}`);
   const z = maybeZ(options);
   const [X, setX] = maybeColumn(x);
   const [Y1, setY1] = column(y);

test/plots/index.ts

@@ -208,6 +208,7 @@ export * from "./penguin-species-island-sex.js";
 export * from "./penguin-species-island.js";
 export * from "./penguin-species.js";
 export * from "./penguin-voronoi-1d.js";
+export * from "./pointer-linked.js";
 export * from "./pointer.js";
 export * from "./polylinear.js";
 export * from "./population-by-latitude.js";

test/plots/pointer-linked.ts

@@ -0,0 +1,16 @@
+import * as Plot from "@observablehq/plot";
+import * as d3 from "d3";
+
+export async function pointerLinkedRectInterval() {
+  const a = d3.range(10).map((i) => ({series: "A", time: i, value: 15 + i - 0.3 * (i * i)}));
+  const b = d3.range(12).map((i) => ({series: "B", time: i, value: 12 * i - i * i}));
+  const round = {floor: (x) => Math.floor(x) - 0.5, offset: (x) => x + 1};
+  const series = [...a, ...b];
+  return Plot.plot({
+    marks: [
+      Plot.rect(series, Plot.pointerX({x: "time", interval: round, fillOpacity: 0.1})),
+      Plot.lineY(series, {stroke: "series", x: "time", y: "value", marker: true, curve: "natural"}),
+      Plot.arrow(series, Plot.pointerX(Plot.groupX({y1: "min", y2: "max"}, {x: "time", y: "value", inset: 10})))
+    ]
+  });
+}