Support for custom marker symbols

[gatopeich]

This pull request introduces support for custom marker functions in Plotly.js scatter plots, allowing users to pass a function directly as the marker.symbol property to define custom SVG marker shapes.

Function Signature

Custom marker functions receive a simple signature:

function customMarker(r, customdata) {
    // r: radius (half of marker.size)
    // customdata: value from trace.customdata[i] for this point
    return 'M...Z';  // SVG path centered at (0,0)
}

• Simple markers can use just (r): function diamond(r) { return 'M' + r + ',0L0,' + r + 'Z'; }
• Data-aware markers use (r, customdata) to vary shape based on data
• Rotation is handled automatically via marker.angle - functions return unrotated paths

Demo

Weather map with sun, cloud, and wind icons generated by custom marker functions:

Live demo: https://gatopeich.github.io/plotly.js/

Changes

• src/components/drawing/index.js: Handle functions as marker.symbol, passing (r, customdata) to custom functions and applying rotation/standoff via align()
• src/components/drawing/symbol_defs.js: Export align() function
• src/lib/coerce.js: Allow functions as valid symbol values
• CUSTOM_MARKER_FUNCTIONS.md: Documentation with examples
• devtools/demos/: Demo files (weather map, backward compatibility, custom markers)
• test/jasmine/tests/drawing_test.js: Tests for custom marker functions

[camdecoster]

Hello @gatopeich! Before we review this PR, could you please help us understand what use case prompted this change? Ideally, you'd create an issue describing the problem and explaining why it should be fixed.

[gatopeich]

I've used this professionally over the last few years to plot meaningful icons on top of maps, e.g. pedestrians, cars, directional wind arrows based on data...
Copilot was able to recreate the concept but did not produce a meaningful demo.

[gatopeich]

Let me add some nice example.
I also realize that the function could take more inputs that the size... I solved that with scoped lambdas in the past but now we can make it ellegant.

[camdecoster]

Sounds good. While you're at it, could you double check the test you added? It's failing in CI.

[gatopeich]

I polished the implementation and added access to customdata.
See what you can do with my custom markers now:

live demo here: https://gatopeich.github.io/plotly.js/demos/weather_map_demo.html