marker

README.md

@@ -949,7 +949,7 @@ 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. 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/master/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 arrow head) 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/master/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*)
 
@@ -992,7 +992,7 @@ For vertical or horizontal links, the **x** option can be specified as shorthand
 
 The link mark supports the [standard mark options](#marks). The **stroke** defaults to currentColor. The **fill** defaults to none. The **strokeWidth** and **strokeMiterlimit** default to one.
 
-The link mark supports [curve options](#curves) to control interpolation between points. Since a link always has two points by definition, only the following curves (or a custom curve) are recommended: *linear*, *step*, *step-after*, *step-before*, *bump-x*, or *bump-y*. Note that the *linear* curve is incapable of showing a fill since a straight line has zero area. For a curved link, you can use a bent [arrow](#arrow) (with no arrowhead, if desired).
+The link mark supports [curve options](#curves) to control interpolation between points, and [marker options](#markers) to add a marker (such as a dot or an arrow head) on each of the control points. Since a link always has two points by definition, only the following curves (or a custom curve) are recommended: *linear*, *step*, *step-after*, *step-before*, *bump-x*, or *bump-y*. Note that the *linear* curve is incapable of showing a fill since a straight line has zero area. For a curved link, you can use a bent [arrow](#arrow) (with no arrowhead, if desired).
 
 #### Plot.link(*data*, *options*)
 
@@ -1896,6 +1896,26 @@ If *curve* is a function, it will be invoked with a given *context* in the same
 
 The tension option only has an effect on cardinal and Catmull–Rom splines (*cardinal*, *cardinal-open*, *cardinal-closed*, *catmull-rom*, *catmull-rom-open*, and *catmull-rom-closed*). For cardinal splines, it corresponds to [tension](https://github.com/d3/d3-shape/blob/master/README.md#curveCardinal_tension); for Catmull–Rom splines, [alpha](https://github.com/d3/d3-shape/blob/master/README.md#curveCatmullRom_alpha).
 
+## Markers
+
+A [marker](https://developer.mozilla.org/en-US/docs/Web/SVG/Element/marker) defines a graphic drawn on vertices of a [line](#line) or a [link](#link) mark. The supported marker options are:
+
+* **markerStart** - the marker for the starting point of a line segment
+* **markerMid** - the marker for any intermediate point of a line segment
+* **markerEnd** - the marker for the end point of a line segment
+* **marker** - shorthand for setting the marker on all points
+
+The following named markers are supported:
+
+* *none* - no marker (default)
+* *arrow* - an arrowhead
+* *circle*, equivalent to *circle-fill* - a filled circle with a white stroke and 3px radius
+* *circle-stroke* - a hollow circle with a colored stroke and a white fill and 3px radius
+
+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).
+
 ## Formats
 
 These helper functions are provided for use as a *scale*.tickFormat [axis option](#position-options), as the text option for [Plot.text](#plottextdata-options), or for general use. See also [d3-format](https://github.com/d3/d3-format), [d3-time-format](https://github.com/d3/d3-time-format), and JavaScript’s built-in [date formatting](https://observablehq.com/@mbostock/date-formatting) and [number formatting](https://observablehq.com/@mbostock/number-formatting).

src/marks/line.js

@@ -4,6 +4,7 @@ import {defined} from "../defined.js";
 import {Mark} from "../plot.js";
 import {indexOf, identity, maybeTuple, maybeZ} from "../options.js";
 import {applyDirectStyles, applyIndirectStyles, applyTransform, applyGroupedChannelStyles, offset} from "../style.js";
+import {applyGroupedMarkers, markers} from "./marker.js";
 
 const defaults = {
   ariaLabel: "line",
@@ -27,6 +28,7 @@ export class Line extends Mark {
       defaults
     );
     this.curve = Curve(curve, tension);
+    markers(this, options);
   }
   render(I, {x, y}, channels) {
     const {x: X, y: Y, z: Z} = channels;
@@ -39,6 +41,7 @@ export class Line extends Mark {
           .join("path")
             .call(applyDirectStyles, this)
             .call(applyGroupedChannelStyles, this, channels)
+            .call(applyGroupedMarkers, this, channels)
             .attr("d", shapeLine()
               .curve(this.curve)
               .defined(i => defined(X[i]) && defined(Y[i]))

src/marks/link.js

@@ -2,6 +2,7 @@ import {create, path} from "d3";
 import {Curve} from "../curve.js";
 import {Mark} from "../plot.js";
 import {applyChannelStyles, applyDirectStyles, applyIndirectStyles, applyTransform, offset} from "../style.js";
+import {markers, applyMarkers} from "./marker.js";
 
 const defaults = {
   ariaLabel: "link",
@@ -25,6 +26,7 @@ export class Link extends Mark {
       defaults
     );
     this.curve = Curve(curve, tension);
+    markers(this, options);
   }
   render(index, {x, y}, channels) {
     const {x1: X1, y1: Y1, x2: X2 = X1, y2: Y2 = Y1} = channels;
@@ -45,7 +47,8 @@ export class Link extends Mark {
               c.lineEnd();
               return p;
             })
-            .call(applyChannelStyles, this, channels))
+            .call(applyChannelStyles, this, channels)
+            .call(applyMarkers, this, channels))
       .node();
   }
 }

src/marks/marker.js

@@ -0,0 +1,98 @@
+import {create} from "d3";
+
+export function markers(mark, {
+  marker,
+  markerStart = marker,
+  markerMid = marker,
+  markerEnd = marker
+} = {}) {
+  mark.markerStart = maybeMarker(markerStart);
+  mark.markerMid = maybeMarker(markerMid);
+  mark.markerEnd = maybeMarker(markerEnd);
+}
+
+function maybeMarker(marker) {
+  if (marker == null || marker === false) return null;
+  if (marker === true) return markerCircleFill;
+  if (typeof marker === "function") return marker;
+  switch (`${marker}`.toLowerCase()) {
+    case "none": return null;
+    case "arrow": return markerArrow;
+    case "circle": case "circle-fill": return markerCircleFill;
+    case "circle-stroke": return markerCircleStroke;
+  }
+  throw new Error(`invalid marker: ${marker}`);
+}
+
+function markerArrow(color) {
+  return create("svg:marker")
+      .attr("viewBox", "-5 -5 10 10")
+      .attr("markerWidth", 6.67)
+      .attr("markerHeight", 6.67)
+      .attr("orient", "auto")
+      .attr("fill", "none")
+      .attr("stroke", color)
+      .attr("stroke-width", 1.5)
+      .attr("stroke-linecap", "round")
+      .attr("stroke-linejoin", "round")
+      .call(marker => marker.append("path").attr("d", "M-1.5,-3l3,3l-3,3"))
+    .node();
+}
+
+function markerCircleFill(color) {
+  return create("svg:marker")
+      .attr("viewBox", "-5 -5 10 10")
+      .attr("markerWidth", 6.67)
+      .attr("markerHeight", 6.67)
+      .attr("fill", color)
+      .attr("stroke", "white")
+      .attr("stroke-width", 1.5)
+      .call(marker => marker.append("circle").attr("r", 3))
+    .node();
+}
+
+function markerCircleStroke(color) {
+  return create("svg:marker")
+      .attr("viewBox", "-5 -5 10 10")
+      .attr("markerWidth", 6.67)
+      .attr("markerHeight", 6.67)
+      .attr("fill", "white")
+      .attr("stroke", color)
+      .attr("stroke-width", 1.5)
+      .call(marker => marker.append("circle").attr("r", 3))
+    .node();
+}
+
+let nextMarkerId = 0;
+
+export function applyMarkers(path, mark, {stroke: S}) {
+  return applyMarkersColor(path, mark, S && (i => S[i]));
+}
+
+export function applyGroupedMarkers(path, mark, {stroke: S}) {
+  return applyMarkersColor(path, mark, S && (([i]) => S[i]));
+}
+
+function applyMarkersColor(path, {markerStart, markerMid, markerEnd, stroke}, strokeof = () => stroke) {
+  const iriByMarkerColor = new Map();
+
+  function applyMarker(marker) {
+    return function(i) {
+      const color = strokeof(i);
+      let iriByColor = iriByMarkerColor.get(marker);
+      if (!iriByColor) iriByMarkerColor.set(marker, iriByColor = new Map());
+      let iri = iriByColor.get(color);
+      if (!iri) {
+        const node = this.parentNode.insertBefore(marker(color), this);
+        const id = `plot-marker-${++nextMarkerId}`;
+        node.setAttribute("id", id);
+        iriByColor.set(color, iri = `url(#${id})`);
+      }
+      return iri;
+    };
+  }
+
+  if (markerStart) path.attr("marker-start", applyMarker(markerStart));
+  if (markerMid) path.attr("marker-mid", applyMarker(markerMid));
+  if (markerEnd) path.attr("marker-end", applyMarker(markerEnd));
+}