More composable forces, such as charge colors. about d3-force

[altsheets]

Oh ... as a workaround, I could start with simply giving the assets a larger charge, right?

But that would also repel the shareholders from that asset.

[mbostock]

I like this idea.

Currently charge is a global force: it applies between all nodes. You can very the strength of the charge per node but that’s it.

If we had charge “groups” (or charge “types” or “colors”) then it would be possible to compose multiple charge forces to different subsets of a graph. For example, red nodes could attract other red nodes, and blue nodes could repel other blue nodes, but red and blue nodes would not interact.

More generally, it might be worth restructuring the force layout so that it’s just position verlet integration, and then you can compose different forces and constraints on top of the simulation: charges, gravity, links, etc. Then you can customize those forces a bit more, such as having a charge force only apply to a subset of a graph.

You can implement this today by copying the appropriate code out of the force layout and applying it yourself inside the tick event. But it would be nice to make such a pattern easier to specify.

[mbostock]

The composable aspect of force layout is now implemented, although I think the API needs some work. In the current implementation, d3.forceSimulation doesn’t provide any forces by default; it only implements alpha decay (for use by listeners) and velocity Verlet integration with decay (friction). For example, to compose the standard force-directed graph layout:

var simulation = d3.forceSimulation(nodes)
    .on("beforetick.charge", d3.forceManyBody(nodes))
    .on("beforetick.link", d3.forceLink(nodes, links))
    .on("beforetick.position", d3.forcePosition(nodes).position([width / 2, height / 2]))
    .on("tick", draw);

I’m going to file a few related issues that cover other specific aspects of the API that I’d like to improve.