src/sankeyCircular.js
function find (nodeById, id) {
    var node = nodeById.get(id)
    if (!node) throw new Error('missing: ' + id)
    return node
  }
function find (nodeById, id) {
    var node = nodeById.get(id)
    if (!node) throw new Error('missing: ' + id)
    return node
  }
Return the node from the collection that matches the provided ID, or throw an error if no match