diff --git a/README.md b/README.md
index fec2d4699b..a0acc8de91 100644
--- a/README.md
+++ b/README.md
@@ -1403,7 +1403,7 @@ The **fill** defaults to none. The **stroke** defaults to currentColor if the fi
 
 Points along the line are connected in input order. Likewise, if there are multiple series via the *z*, *fill*, or *stroke* channel, the series are drawn in input order such that the last series is drawn on top. Typically, the data is already in sorted order, such as chronological for time series; if sorting is needed, consider a [sort transform](#transforms).
 
-The line mark supports [curve options](#curves) to control interpolation between points, and [marker options](#markers) to add a marker (such as a dot or an arrowhead) on each of the control points. If any of the *x* or *y* values are invalid (undefined, null, or NaN), the line will be interrupted, resulting in a break that divides the line shape into multiple segments. (See [d3-shape’s *line*.defined](https://github.com/d3/d3-shape/blob/main/README.md#line_defined) for more.) If a line segment consists of only a single point, it may appear invisible unless rendered with rounded or square line caps. In addition, some curves such as *cardinal-open* only render a visible segment if it contains multiple points.
+The line mark supports [curve options](#curves) to control interpolation between points, and [marker options](#markers) to add a marker (such as a dot or an arrowhead) on each of the control points. The default curve is *auto*, which is equivalent to *linear* if there is no [projection](#projection-options), and otherwise uses the associated projection. If any of the *x* or *y* values are invalid (undefined, null, or NaN), the line will be interrupted, resulting in a break that divides the line shape into multiple segments. (See [d3-shape’s *line*.defined](https://github.com/d3/d3-shape/blob/main/README.md#line_defined) for more.) If a line segment consists of only a single point, it may appear invisible unless rendered with rounded or square line caps. In addition, some curves such as *cardinal-open* only render a visible segment if it contains multiple points.
 
 #### Plot.line(*data*, *options*)
 
@@ -2836,9 +2836,9 @@ The following named curve methods are supported:
 * *step* - a piecewise constant function where *y* changes at the midpoint of *x*
 * *step-after* - a piecewise constant function where *y* changes after *x*
 * *step-before* - a piecewise constant function where *x* changes after *y*
-* *projected* - use the (possibly spherical) [projection](#projection-options)
+* *auto* - like *linear*, but use the (possibly spherical) [projection](#projection-options), if any
 
-If *curve* is a function, it will be invoked with a given *context* in the same fashion as a [D3 curve factory](https://github.com/d3/d3-shape/blob/main/README.md#custom-curves). The *projected* curve is only available for the [line mark](#line) and is typically used in conjunction with a spherical [projection](#projection-options) to interpolate along [geodesics](https://en.wikipedia.org/wiki/Geodesic).
+If *curve* is a function, it will be invoked with a given *context* in the same fashion as a [D3 curve factory](https://github.com/d3/d3-shape/blob/main/README.md#custom-curves). The *auto* curve is only available for the [line mark](#line) and is typically used in conjunction with a spherical [projection](#projection-options) to interpolate along [geodesics](https://en.wikipedia.org/wiki/Geodesic).
 
 The tension option only has an effect on bundle, cardinal and Catmull–Rom splines (*bundle*, *cardinal*, *cardinal-open*, *cardinal-closed*, *catmull-rom*, *catmull-rom-open*, and *catmull-rom-closed*). For bundle splines, it corresponds to [beta](https://github.com/d3/d3-shape/blob/main/README.md#curveBundle_beta); for cardinal splines, [tension](https://github.com/d3/d3-shape/blob/main/README.md#curveCardinal_tension); for Catmull–Rom splines, [alpha](https://github.com/d3/d3-shape/blob/main/README.md#curveCatmullRom_alpha).
 
@@ -2861,7 +2861,7 @@ The following named markers are supported:
 
 If *marker* is true, it defaults to *circle*. If *marker* is a function, it will be called with a given *color* and must return an SVG marker element.
 
-The primary color of a marker is inherited from the *stroke* of the associated mark. The *arrow* marker is [automatically oriented](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/orient) such that it points in the tangential direction of the path at the position the marker is placed. The *circle* markers are centered around the given vertex. Note that lines whose curve is not *linear* (the default), markers are not necessarily drawn at the data positions given by *x* and *y*; marker placement is determined by the (possibly Bézier) path segments generated by the curve. To ensure that symbols are drawn at a given *x* and *y* position, consider using a [dot](#dot).
+The primary color of a marker is inherited from the *stroke* of the associated mark. The *arrow* marker is [automatically oriented](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/orient) such that it points in the tangential direction of the path at the position the marker is placed. The *circle* markers are centered around the given vertex. Note that for lines whose curve is not *linear*, markers are not necessarily drawn at the data positions given by *x* and *y*; marker placement is determined by the (possibly Bézier) path segments generated by the curve. To ensure that symbols are drawn at a given *x* and *y* position, consider using a [dot](#dot).
 
 ## Formats
 
diff --git a/src/channel.js b/src/channel.js
index eb8eec1fd8..2a5bdc37e6 100644
--- a/src/channel.js
+++ b/src/channel.js
@@ -1,6 +1,5 @@
 import {ascending, descending, rollup, sort} from "d3";
 import {first, isIterable, labelof, map, maybeValue, range, valueof} from "./options.js";
-import {maybeApplyProjection} from "./projection.js";
 import {registry} from "./scales/index.js";
 import {maybeReduce} from "./transforms/group.js";
 
@@ -25,8 +24,8 @@ export function Channels(descriptors, data) {
 }
 
 // TODO Use Float64Array for scales with numeric ranges, e.g. position?
-export function valueObject(channels, scales, {projection}) {
-  const values = Object.fromEntries(
+export function valueObject(channels, scales) {
+  return Object.fromEntries(
     Object.entries(channels).map(([name, {scale: scaleName, value}]) => {
       let scale;
       if (scaleName !== undefined) {
@@ -35,22 +34,6 @@ export function valueObject(channels, scales, {projection}) {
       return [name, scale === undefined ? value : map(value, scale)];
     })
   );
-
-  // If there is a projection, and there are both x and y channels (or x1 and
-  // y1, or x2 andy2 channels), and those channels are associated with the x and
-  // y scale respectively (and not already in screen coordinates as with an
-  // initializer), then apply the projection, replacing the x and y values. Note
-  // that the x and y scales themselves don’t exist if there is a projection,
-  // but whether the channels are associated with scales still determines
-  // whether the projection should apply; think of the projection as a
-  // combination xy-scale.
-  if (projection) {
-    maybeApplyProjection("x", "y", channels, values, projection);
-    maybeApplyProjection("x1", "y1", channels, values, projection);
-    maybeApplyProjection("x2", "y2", channels, values, projection);
-  }
-
-  return values;
 }
 
 // Note: mutates channel.domain! This is set to a function so that it is lazily
diff --git a/src/marks/density.js b/src/marks/density.js
index eec57de311..c862a0d540 100644
--- a/src/marks/density.js
+++ b/src/marks/density.js
@@ -2,6 +2,7 @@ import {contourDensity, create, geoPath} from "d3";
 import {valueObject} from "../channel.js";
 import {isTypedArray, maybeTuple, maybeZ} from "../options.js";
 import {Mark} from "../plot.js";
+import {maybeProject} from "../projection.js";
 import {coerceNumbers} from "../scales.js";
 import {
   applyFrameAnchor,
@@ -92,18 +93,15 @@ function densityInitializer(options, fillDensity, strokeDensity) {
     const [cx, cy] = applyFrameAnchor(this, dimensions);
     const {width, height} = dimensions;
 
-    // Extract the scaled (or projected!) values for the x and y channels.
-    let {x: X, y: Y} = valueObject(
-      {
-        ...(channels.x && {x: channels.x}),
-        ...(channels.y && {y: channels.y})
-      },
-      scales,
-      context
-    );
+    // Extract the (possibly) scaled values for the x and y channels.
+    const position = valueObject({...(channels.x && {x: channels.x}), ...(channels.y && {y: channels.y})}, scales);
+
+    // Apply the projection.
+    if (context.projection) maybeProject("x", "y", channels, position, context);
 
     // Coerce the x and y channels to numbers (so that null is properly treated
     // as an undefined value rather than being coerced to zero).
+    let {x: X, y: Y} = position;
     if (X) X = coerceNumbers(X);
     if (Y) Y = coerceNumbers(Y);
 
diff --git a/src/marks/line.js b/src/marks/line.js
index 53a9525589..95bc1c416d 100644
--- a/src/marks/line.js
+++ b/src/marks/line.js
@@ -1,4 +1,4 @@
-import {geoPath, line as shapeLine} from "d3";
+import {curveLinear, geoPath, line as shapeLine} from "d3";
 import {create} from "../context.js";
 import {Curve} from "../curve.js";
 import {indexOf, identity, maybeTuple, maybeZ} from "../options.js";
@@ -24,37 +24,45 @@ const defaults = {
   strokeMiterlimit: 1
 };
 
-const curveProjected = Symbol("projected");
+// This is a special built-in curve that will use d3.geoPath when there is a
+// projection, and the linear curve when there is not. You can explicitly
+// opt-out of d3.geoPath and instead use d3.line with the "linear" curve.
+function curveAuto(context) {
+  return curveLinear(context);
+}
 
-// For the “projected” curve, return a symbol instead of a curve
-// implementation; we’ll use d3.geoPath instead of d3.line to render.
-function LineCurve({curve, tension}) {
-  return typeof curve !== "function" && `${curve}`.toLowerCase() === "projected"
-    ? curveProjected
-    : Curve(curve, tension);
+// For the “auto” curve, return a symbol instead of a curve implementation;
+// we’ll use d3.geoPath instead of d3.line to render if there’s a projection.
+function LineCurve({curve = curveAuto, tension}) {
+  return typeof curve !== "function" && `${curve}`.toLowerCase() === "auto" ? curveAuto : Curve(curve, tension);
 }
 
 export class Line extends Mark {
   constructor(data, options = {}) {
     const {x, y, z} = options;
-    const curve = LineCurve(options);
     super(
       data,
       {
-        x: {value: x, scale: curve === curveProjected ? undefined : "x"}, // unscaled if projected
-        y: {value: y, scale: curve === curveProjected ? undefined : "y"}, // unscaled if projected
+        x: {value: x, scale: "x"},
+        y: {value: y, scale: "y"},
         z: {value: maybeZ(options), optional: true}
       },
       options,
       defaults
     );
     this.z = z;
-    this.curve = curve;
+    this.curve = LineCurve(options);
     markers(this, options);
   }
   filter(index) {
     return index;
   }
+  project(channels, values, context) {
+    // For the auto curve, projection is handled at render.
+    if (this.curve !== curveAuto) {
+      super.project(channels, values, context);
+    }
+  }
   render(index, scales, channels, dimensions, context) {
     const {x: X, y: Y} = channels;
     const {curve} = this;
@@ -72,7 +80,7 @@ export class Line extends Mark {
           .call(applyGroupedMarkers, this, channels)
           .attr(
             "d",
-            curve === curveProjected
+            curve === curveAuto && context.projection
               ? sphereLine(context.projection, X, Y)
               : shapeLine()
                   .curve(curve)
diff --git a/src/plot.js b/src/plot.js
index badf692b7e..f5adcfdb34 100644
--- a/src/plot.js
+++ b/src/plot.js
@@ -17,6 +17,7 @@ import {
   where,
   yes
 } from "./options.js";
+import {maybeProject} from "./projection.js";
 import {Scales, ScaleFunctions, autoScaleRange, exposeScales} from "./scales.js";
 import {position, registry as scaleRegistry} from "./scales/index.js";
 import {applyInlineStyles, maybeClassName, maybeClip, styles} from "./style.js";
@@ -169,9 +170,16 @@ export function plot(options = {}) {
 
   autoScaleLabels(channelsByScale, scaleDescriptors, axes, dimensions, options);
 
-  // Compute value objects, applying scales and projection as needed.
+  // Compute value objects, applying scales as needed.
   for (const state of stateByMark.values()) {
-    state.values = valueObject(state.channels, scales, context);
+    state.values = valueObject(state.channels, scales);
+  }
+
+  // Apply projection as needed.
+  if (context.projection) {
+    for (const [mark, state] of stateByMark) {
+      mark.project(state.channels, state.values, context);
+    }
   }
 
   const {width, height} = dimensions;
@@ -367,6 +375,19 @@ export class Mark {
     }
     return index;
   }
+  // If there is a projection, and there are both x and y channels (or x1 and
+  // y1, or x2 and y2 channels), and those channels are associated with the x
+  // and y scale respectively (and not already in screen coordinates as with an
+  // initializer), then apply the projection, replacing the x and y values. Note
+  // that the x and y scales themselves don’t exist if there is a projection,
+  // but whether the channels are associated with scales still determines
+  // whether the projection should apply; think of the projection as a
+  // combination xy-scale.
+  project(channels, values, context) {
+    maybeProject("x", "y", channels, values, context);
+    maybeProject("x1", "y1", channels, values, context);
+    maybeProject("x2", "y2", channels, values, context);
+  }
   plot({marks = [], ...options} = {}) {
     return plot({...options, marks: [...marks, this]});
   }
diff --git a/src/projection.js b/src/projection.js
index 404f62b87b..00ef46fced 100644
--- a/src/projection.js
+++ b/src/projection.js
@@ -203,11 +203,12 @@ function conicProjection(createProjection, kx, ky) {
 }
 
 // Applies a point-wise projection to the given paired x and y channels.
-export function maybeApplyProjection(cx, cy, channels, values, projection) {
+// Note: mutates values!
+export function maybeProject(cx, cy, channels, values, context) {
   const x = channels[cx] && channels[cx].scale === "x";
   const y = channels[cy] && channels[cy].scale === "y";
   if (x && y) {
-    applyProjection(cx, cy, values, projection);
+    project(cx, cy, values, context.projection);
   } else if (x) {
     throw new Error(`projection requires paired x and y channels; ${cx} is missing ${cy}`);
   } else if (y) {
@@ -215,7 +216,7 @@ export function maybeApplyProjection(cx, cy, channels, values, projection) {
   }
 }
 
-function applyProjection(cx, cy, values, projection) {
+function project(cx, cy, values, projection) {
   const x = values[cx];
   const y = values[cy];
   const n = x.length;
diff --git a/src/transforms/hexbin.js b/src/transforms/hexbin.js
index 3f880dff30..1b7af7f952 100644
--- a/src/transforms/hexbin.js
+++ b/src/transforms/hexbin.js
@@ -4,6 +4,7 @@ import {sqrt3} from "../symbols.js";
 import {isNoneish, number, valueof} from "../options.js";
 import {initializer} from "./basic.js";
 import {hasOutput, maybeGroup, maybeOutputs, maybeSubgroup} from "./group.js";
+import {maybeProject} from "../projection.js";
 
 // We don’t want the hexagons to align with the edges of the plot frame, as that
 // would cause extreme x-values (the upper bound of the default x-scale domain)
@@ -31,15 +32,20 @@ export function hexbin(outputs = {fill: "count"}, {binWidth, ...options} = {}) {
   if (options.symbol === undefined) options.symbol = "hexagon";
   if (options.r === undefined && !hasOutput(outputs, "r")) options.r = binWidth / 2;
 
-  return initializer(options, (data, facets, {x: X, y: Y, z: Z, fill: F, stroke: S, symbol: Q}, scales, _, context) => {
+  return initializer(options, (data, facets, channels, scales, _, context) => {
+    let {x: X, y: Y, z: Z, fill: F, stroke: S, symbol: Q} = channels;
     if (X === undefined) throw new Error("missing channel: x");
     if (Y === undefined) throw new Error("missing channel: y");
 
-    // Extract the scaled (or projected!) values for the x and y channels.
-    ({x: X, y: Y} = valueObject({x: X, y: Y}, scales, context));
+    // Extract the (possibly) scaled values for the x and y channels.
+    const position = valueObject({x: X, y: Y}, scales);
+
+    // Apply the projection.
+    if (context.projection) maybeProject("x", "y", channels, position, context);
 
     // Coerce the x and y channels to numbers (so that null is properly
     // treated as an undefined value rather than being coerced to zero).
+    ({x: X, y: Y} = position);
     X = coerceNumbers(X);
     Y = coerceNumbers(Y);
 
@@ -84,7 +90,7 @@ export function hexbin(outputs = {fill: "count"}, {binWidth, ...options} = {}) {
     }
 
     // Construct the output channels, and populate the radius scale hint.
-    const channels = {
+    const binChannels = {
       x: {value: BX},
       y: {value: BY},
       ...(Z && {z: {value: GZ}}),
@@ -99,7 +105,7 @@ export function hexbin(outputs = {fill: "count"}, {binWidth, ...options} = {}) {
       )
     };
 
-    return {data, facets: binFacets, channels};
+    return {data, facets: binFacets, channels: binChannels};
   });
 }
 
diff --git a/test/marks/line-test.js b/test/marks/line-test.js
index 052925fa9a..494337420a 100644
--- a/test/marks/line-test.js
+++ b/test/marks/line-test.js
@@ -1,5 +1,5 @@
 import * as Plot from "@observablehq/plot";
-import {curveLinear, curveStep} from "d3";
+import {curveStep} from "d3";
 import assert from "assert";
 
 it("line() has the expected defaults", () => {
@@ -26,7 +26,7 @@ it("line() has the expected defaults", () => {
     Object.values(line.channels).map((c) => c.scale),
     ["x", "y"]
   );
-  assert.strictEqual(line.curve, curveLinear);
+  assert.strictEqual(line.curve.name, "curveAuto");
   assert.strictEqual(line.fill, "none");
   assert.strictEqual(line.fillOpacity, undefined);
   assert.strictEqual(line.stroke, "currentColor");
diff --git a/test/plots/beagle.js b/test/plots/beagle.js
index c5219f9b7f..d5d34b43eb 100644
--- a/test/plots/beagle.js
+++ b/test/plots/beagle.js
@@ -15,7 +15,7 @@ export default async function () {
     marks: [
       Plot.geo(land, {fill: "currentColor"}),
       Plot.graticule(),
-      Plot.line(beagle, {stroke: (d, i) => i, z: null, curve: "projected"}),
+      Plot.line(beagle, {stroke: (d, i) => i, z: null}),
       Plot.sphere()
     ]
   });