Add comprehensive documentation for custom marker API

Co-authored-by: gatopeich <7722268+gatopeich@users.noreply.github.com>
plotly · gatopeich · Nov 19, 2025 · Nov 19, 2025 · Nov 19, 2025 · Nov 19, 2025
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+# Custom Marker API
+
+This document describes the custom marker API added to plotly.js that allows users to register custom SVG marker symbols dynamically.
+
+## Overview
+
+The custom marker API enables developers to extend plotly.js with their own marker shapes beyond the built-in symbols. Custom markers work seamlessly with all existing marker features including size, color, line styling, and automatic variants.
+
+## API Reference
+
+### `Plotly.Drawing.addCustomMarker(name, drawFunc, opts)`
+
+Registers a new custom marker symbol.
+
+#### Parameters
+
+- **name** (string, required): The name of the new marker symbol. This name will be used to reference the marker in plots (e.g., `marker.symbol: 'mymarker'`).
+
+- **drawFunc** (function, required): A function that generates the SVG path string for the marker. The function receives three parameters:
+  - `r` (number): The radius/size of the marker
+  - `angle` (number): The rotation angle in degrees (for directional markers)
+  - `standoff` (number): The standoff distance from the point
+
+  The function should return a valid SVG path string (e.g., `"M0,0L10,0L5,8.66Z"`).
+
+- **opts** (object, optional): Configuration options:
+  - `backoff` (number): Backoff distance for this symbol when used with lines. Default: 0
+  - `needLine` (boolean): Whether this symbol requires a line stroke. Default: false
+  - `noDot` (boolean): If true, skips creating `-dot` and `-open-dot` variants. Default: false
+  - `noFill` (boolean): If true, the symbol should not be filled. Default: false
+
+#### Returns
+
+- (number): The symbol number assigned to the new marker. Returns the existing symbol number if the marker name is already registered.
+
+#### Marker Variants
+
+Unless `opts.noDot` is true, the following variants are automatically created:
+
+- `name`: Base marker (filled)
+- `name-open`: Open marker (outline only, no fill)
+- `name-dot`: Base marker with a dot in the center
+- `name-open-dot`: Open marker with a dot in the center
+
+## Examples
+
+### Basic Heart-Shaped Marker
+
+```javascript
+// Define the marker path function
+function heartMarker(r, angle, standoff) {
+    var x = r * 0.6;
+    var y = r * 0.8;
+    return 'M0,' + (-y/2) + 
+           'C' + (-x) + ',' + (-y) + ' ' + (-x*2) + ',' + (-y/3) + ' ' + (-x*2) + ',0' +
+           'C' + (-x*2) + ',' + (y/2) + ' 0,' + (y) + ' 0,' + (y*1.5) +
+           'C0,' + (y) + ' ' + (x*2) + ',' + (y/2) + ' ' + (x*2) + ',0' +
+           'C' + (x*2) + ',' + (-y/3) + ' ' + (x) + ',' + (-y) + ' 0,' + (-y/2) + 'Z';
+}
+
+// Register the marker
+Plotly.Drawing.addCustomMarker('heart', heartMarker);
+
+// Use it in a plot
+Plotly.newPlot('myDiv', [{
+    type: 'scatter',
+    x: [1, 2, 3, 4, 5],
+    y: [2, 3, 4, 3, 2],
+    mode: 'markers',
+    marker: {
+        symbol: 'heart',
+        size: 15,
+        color: 'red'
+    }
+}]);
+```
+
+### 5-Point Star Marker
+
+```javascript
+function star5Marker(r, angle, standoff) {
+    var points = 5;
+    var outerRadius = r;
+    var innerRadius = r * 0.4;
+    var path = 'M';
+
+    for (var i = 0; i < points * 2; i++) {
+        var radius = i % 2 === 0 ? outerRadius : innerRadius;
+        var ang = (i * Math.PI) / points - Math.PI / 2;
+        var x = radius * Math.cos(ang);
+        var y = radius * Math.sin(ang);
+        path += (i === 0 ? '' : 'L') + x.toFixed(2) + ',' + y.toFixed(2);
+    }
+    path += 'Z';
+    return path;
+}
+
+Plotly.Drawing.addCustomMarker('star5', star5Marker);
+
+Plotly.newPlot('myDiv', [{
+    type: 'scatter',
+    x: [1, 2, 3, 4, 5],
+    y: [2, 3, 4, 3, 2],
+    mode: 'markers',
+    marker: {
+        symbol: 'star5',
+        size: 18,
+        color: 'gold'
+    }
+}]);
+```
+
+### Using Marker Variants
+
+```javascript
+// Once registered, all variants are available
+Plotly.newPlot('myDiv', [{
+    type: 'scatter',
+    x: [1, 2, 3, 4],
+    y: [1, 2, 3, 4],
+    mode: 'markers',
+    marker: {
+        symbol: ['heart', 'heart-open', 'heart-dot', 'heart-open-dot'],
+        size: 15,
+        color: ['red', 'pink', 'crimson', 'lightcoral']
+    }
+}]);
+```
+
+### Line Marker with Options
+
+```javascript
+// Custom line marker that doesn't need dot variants
+function horizontalLine(r, angle, standoff) {
+    return 'M-' + r + ',0L' + r + ',0';
+}
+
+Plotly.Drawing.addCustomMarker('hline', horizontalLine, {
+    noDot: true,     // Don't create -dot variants
+    needLine: true,  // This marker needs stroke
+    noFill: true     // This marker should not be filled
+});
+
+Plotly.newPlot('myDiv', [{
+    type: 'scatter',
+    x: [1, 2, 3],
+    y: [2, 3, 4],
+    mode: 'markers',
+    marker: {
+        symbol: 'hline',
+        size: 15,
+        line: { color: 'blue', width: 2 }
+    }
+}]);
+```
+
+### Arrow Marker with Backoff
+
+```javascript
+// Custom arrow marker with backoff for better line connection
+function arrowMarker(r, angle, standoff) {
+    var headAngle = Math.PI / 4;
+    var x = 2 * r * Math.cos(headAngle);
+    var y = 2 * r * Math.sin(headAngle);
+
+    return 'M0,0L' + (-x) + ',' + y + 'L' + x + ',' + y + 'Z';
+}
+
+Plotly.Drawing.addCustomMarker('myarrow', arrowMarker, {
+    backoff: 0.5  // Backoff distance for line connection
+});
+
+Plotly.newPlot('myDiv', [{
+    type: 'scatter',
+    x: [1, 2, 3, 4],
+    y: [1, 2, 3, 4],
+    mode: 'markers+lines',
+    marker: {
+        symbol: 'myarrow',
+        size: 15
+    }
+}]);
+```
+
+## SVG Path Reference
+
+The `drawFunc` should return a valid SVG path string. Here are the common SVG path commands:
+
+- `M x,y`: Move to absolute position (x, y)
+- `m dx,dy`: Move to relative position (dx, dy)
+- `L x,y`: Line to absolute position
+- `l dx,dy`: Line to relative position
+- `H x`: Horizontal line to x
+- `h dx`: Horizontal line by dx
+- `V y`: Vertical line to y
+- `v dy`: Vertical line by dy
+- `C x1,y1 x2,y2 x,y`: Cubic Bézier curve
+- `Q x1,y1 x,y`: Quadratic Bézier curve
+- `A rx,ry rotation large-arc sweep x,y`: Elliptical arc
+- `Z`: Close path
+
+The path should typically:
+- Be centered at (0, 0)
+- Scale proportionally with the radius `r`
+- Return to the start point (close the path with 'Z' for filled shapes)
+
+## Notes
+
+- Custom markers are registered globally and persist for the lifetime of the page
+- Marker names are case-sensitive
+- Attempting to register a marker with the same name twice will return the existing symbol number without creating a duplicate
+- The `angle` and `standoff` parameters are provided for advanced use cases (e.g., directional markers on maps)
+- For most simple shapes, you can ignore the `angle` and `standoff` parameters
+
+## Browser Compatibility
+
+Custom markers work in all browsers that support plotly.js and SVG path rendering.
+
+## Demo
+
+See `devtools/custom_marker_demo.html` for a complete working example.