Support interval+reduce in the area mark

src/marks/area.js

@@ -2,16 +2,10 @@ 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 {
-  applyDirectStyles,
-  applyIndirectStyles,
-  applyTransform,
-  applyGroupedChannelStyles,
-  groupIndex
-} from "../style.js";
+import {first, maybeZ, second} from "../options.js";
+import {applyDirectStyles, applyIndirectStyles, applyTransform, applyGroupedChannelStyles} from "../style.js";
+import {groupIndex} from "../style.js";
 import {maybeDenseIntervalX, maybeDenseIntervalY} from "../transforms/bin.js";
-import {maybeIdentityX, maybeIdentityY} from "../transforms/identity.js";
 import {maybeStackX, maybeStackY} from "../transforms/stack.js";
 
 const defaults = {
@@ -77,11 +71,11 @@ export function area(data, options) {
 }
 
 export function areaX(data, options) {
-  const {y = indexOf, ...rest} = maybeDenseIntervalY(options);
-  return new Area(data, maybeStackX(maybeIdentityX({...rest, y1: y, y2: undefined}, y === indexOf ? "x2" : "x")));
+  const {y, ...rest} = maybeDenseIntervalY(options);
+  return new Area(data, maybeStackX({...rest, y1: y, y2: undefined}));
 }
 
 export function areaY(data, options) {
-  const {x = indexOf, ...rest} = maybeDenseIntervalX(options);
-  return new Area(data, maybeStackY(maybeIdentityY({...rest, x1: x, x2: undefined}, x === indexOf ? "y2" : "y")));
+  const {x, ...rest} = maybeDenseIntervalX(options);
+  return new Area(data, maybeStackY({...rest, x1: x, x2: undefined}));
 }

src/marks/line.js

@@ -3,14 +3,9 @@ import {create} from "../context.js";
 import {curveAuto, maybeCurveAuto} from "../curve.js";
 import {Mark} from "../mark.js";
 import {applyGroupedMarkers, markers} from "../marker.js";
-import {coerceNumbers, indexOf, identity, maybeTuple, maybeZ} from "../options.js";
-import {
-  applyDirectStyles,
-  applyIndirectStyles,
-  applyTransform,
-  applyGroupedChannelStyles,
-  groupIndex
-} from "../style.js";
+import {coerceNumbers, maybeTuple, maybeZ} from "../options.js";
+import {applyDirectStyles, applyIndirectStyles, applyTransform, applyGroupedChannelStyles} from "../style.js";
+import {groupIndex} from "../style.js";
 import {maybeDenseIntervalX, maybeDenseIntervalY} from "../transforms/bin.js";
 
 const defaults = {
@@ -103,10 +98,10 @@ export function line(data, {x, y, ...options} = {}) {
   return new Line(data, {...options, x, y});
 }
 
-export function lineX(data, {x = identity, y = indexOf, ...options} = {}) {
-  return new Line(data, maybeDenseIntervalY({...options, x, y}));
+export function lineX(data, options) {
+  return new Line(data, maybeDenseIntervalY(options));
 }
 
-export function lineY(data, {x = indexOf, y = identity, ...options} = {}) {
-  return new Line(data, maybeDenseIntervalX({...options, x, y}));
+export function lineY(data, options) {
+  return new Line(data, maybeDenseIntervalX(options));
 }

src/transforms/bin.js

@@ -13,6 +13,7 @@ import {
   coerceDate,
   coerceNumbers,
   identity,
+  indexOf,
   isInterval,
   isIterable,
   isTemporal,
@@ -77,12 +78,12 @@ function maybeDenseInterval(bin, k, options = {}) {
   return bin(outputs, options);
 }
 
-export function maybeDenseIntervalX(options = {}) {
-  return maybeDenseInterval(binX, "y", withTip(options, "x"));
+export function maybeDenseIntervalX({y = identity, x = indexOf, ...options} = {}) {
+  return maybeDenseInterval(binX, "y", withTip({x, y, ...options}, "x"));
 }
 
-export function maybeDenseIntervalY(options = {}) {
-  return maybeDenseInterval(binY, "x", withTip(options, "y"));
+export function maybeDenseIntervalY({x = identity, y = indexOf, ...options} = {}) {
+  return maybeDenseInterval(binY, "x", withTip({x, y, ...options}, "y"));
 }
 
 function binn(