function ascendingTargetBreadth (a, b) {
    return ascendingBreadth(a.target, b.target) || a.index - b.index
  }
function ascendingTargetBreadth (a, b) {
    return ascendingBreadth(a.target, b.target) || a.index - b.index
  }
function ascendingTargetBreadth ( a , b ) { return ascendingBreadth ( a . target , b . target ) || a . index - b . index }
sort links' breadth (ie top to bottom in a column), based on their target nodes' breadths
sort links breadth ( ie top to bottom in a column ) based on their target nodes breadths