src/sankeyCircular.js
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