pack.radius()

pack.radius([radius])

If radius is specified, sets the pack layout’s radius accessor to the specified function and returns this pack layout. If radius is not specified, returns the current radius accessor, which defaults to null. If the radius accessor is null, the radius of each leaf circle is derived from the leaf node.value (computed by node.sum); the radii are then scaled proportionally to fit the layout size. If the radius accessor is not null, the radius of each leaf circle is specified exactly by the function.