src/sankeyCircular.js
function ascendingSourceBreadth (a, b) {
    return ascendingBreadth(a.source, b.source) || a.index - b.index
  }
function ascendingSourceBreadth (a, b) {
    return ascendingBreadth(a.source, b.source) || a.index - b.index
  }
sort links' breadth (ie top to bottom in a column), based on their source nodes' breadths