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
  }
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
sort links breadth ( ie top to bottom in a column ) based on their source nodes breadths