add missing test

test/transforms/stack-test.ts

@@ -0,0 +1,19 @@
+import * as Plot from "@observablehq/plot";
+import assert from "assert";
+
+it("stackY(options) returns the expected values", () => {
+  const y = [1, 2, -2, -1];
+  const {
+    channels: {
+      y1: {value: Y1},
+      y2: {value: Y2}
+    }
+  } = (Plot.barY(y, Plot.stackY({y})) as any).initialize();
+  assert.deepStrictEqual(y, [1, 2, -2, -1]);
+  assert.deepStrictEqual(Y1, Float64Array.of(0, 1, 0, -2));
+  assert.deepStrictEqual(Y2, Float64Array.of(1, 3, -2, -3));
+});
+
+it("stackY({order}) rejects an invalid order", () => {
+  assert.throws(() => Plot.barY([], {y: 1, order: 42 as any}), /^Error: invalid order: 42$/);
+});