fix: areaY dense interval without explicit y option (#2328)

Fixed areaY to work with dense intervals (interval + reduce) without
requiring an explicit y option, matching lineY behavior.

🤖 Generated with [Claude Code](https://claude.ai/code)

Total cost:            $5.51
Total duration (API):  23m 31.3s
Total duration (wall): 14h 30m 4.3s
Total code changes:    208 lines added, 53 lines removed
Token usage by model:
    claude-3-5-haiku:  204.7k input, 4.8k output, 0 cache read, 0 cache write
       claude-sonnet:  18.9k input, 28.4k output, 8.3m cache read, 629.5k cache write

Author: claude

src/marks/area.js

@@ -2,7 +2,7 @@ import {area as shapeArea} from "d3";
 import {create} from "../context.js";
 import {maybeCurve} from "../curve.js";
 import {Mark} from "../mark.js";
-import {first, indexOf, maybeZ, second} from "../options.js";
+import {first, identity, indexOf, maybeZ, second} from "../options.js";
 import {
   applyDirectStyles,
   applyIndirectStyles,
@@ -77,11 +77,15 @@ export function area(data, options) {
 }
 
 export function areaX(data, options) {
-  const {y = indexOf, ...rest} = maybeDenseIntervalY(options);
+  const {y = indexOf, ...rest} = maybeDenseIntervalY(
+    options?.interval != null ? {...options, x: options.x ?? identity} : options
+  );
   return new Area(data, maybeStackX(maybeIdentityX({...rest, y1: y, y2: undefined}, y === indexOf ? "x2" : "x")));
 }
 
 export function areaY(data, options) {
-  const {x = indexOf, ...rest} = maybeDenseIntervalX(options);
+  const {x = indexOf, ...rest} = maybeDenseIntervalX(
+    options?.interval != null ? {...options, y: options.y ?? identity} : options
+  );
   return new Area(data, maybeStackY(maybeIdentityY({...rest, x1: x, x2: undefined}, x === indexOf ? "y2" : "y")));
 }

test/output/denseIntervalAreaY.svg

@@ -0,0 +1,16 @@
+import * as Plot from "@observablehq/plot";
+import * as d3 from "d3";
+
+export async function denseIntervalAreaY() {
+  const aapl = await d3.csv<any>("data/aapl.csv", d3.autoType);
+  return Plot.plot({
+    marks: [Plot.areaY(aapl, {x: "Date", reduce: "count", interval: "month"})]
+  });
+}
+
+export async function denseIntervalLineY() {
+  const aapl = await d3.csv<any>("data/aapl.csv", d3.autoType);
+  return Plot.plot({
+    marks: [Plot.lineY(aapl, {x: "Date", reduce: "count", interval: "month"})]
+  });
+}

test/output/denseIntervalLineY.svg

@@ -48,6 +48,7 @@ export * from "./boxplot.js";
 export * from "./caltrain-direction.js";
 export * from "./caltrain.js";
 export * from "./cars-dodge.js";
+export * from "./dense-interval.js";
 export * from "./cars-hexbin.js";
 export * from "./cars-jitter.js";
 export * from "./cars-mpg.js";