tree.separation()

tree.separation([separation])

If separation is specified, sets the separation accessor to the specified function and returns this tree layout. If separation is not specified, returns the current separation accessor, which defaults to:

function separation(a, b) {
  return a.parent == b.parent ? 1 : 2;
}

A variation that is more appropriate for radial layouts reduces the separation gap proportionally to the radius:

function separation(a, b) {
  return (a.parent == b.parent ? 1 : 2) / a.depth;
}

The separation accessor is used to separate neighboring nodes. The separation function is passed two nodes a and b, and must return the desired separation. The nodes are typically siblings, though the nodes may be more distantly related if the layout decides to place such nodes adjacent.