in pytheas-core/src/main/resources/js/d3/d3.layout.js [514:545]
function d3_layout_forceAccumulate(quad, alpha, charges) {
var cx = 0,
cy = 0;
quad.charge = 0;
if (!quad.leaf) {
var nodes = quad.nodes,
n = nodes.length,
i = -1,
c;
while (++i < n) {
c = nodes[i];
if (c == null) continue;
d3_layout_forceAccumulate(c, alpha, charges);
quad.charge += c.charge;
cx += c.charge * c.cx;
cy += c.charge * c.cy;
}
}
if (quad.point) {
// jitter internal nodes that are coincident
if (!quad.leaf) {
quad.point.x += Math.random() - .5;
quad.point.y += Math.random() - .5;
}
var k = alpha * charges[quad.point.index];
quad.charge += quad.pointCharge = k;
cx += k * quad.point.x;
cy += k * quad.point.y;
}
quad.cx = cx / quad.charge;
quad.cy = cy / quad.charge;
}