the Node in document coordinates.

  // The normal mechanism for determining the size of the document depends on all of the
  // Nodes and Links existing, so we need to use a function that depends only on the model data.
  function computeDocumentBounds() {
    const b = new go.Rect();
    const ndata = myWholeModel.nodeDataArray;
    for (let i = 0; i < ndata.length; i++) {
      const d = ndata[i];
      if (!d.bounds) continue;
      if (i === 0) {
        b.set(d.bounds);
      } else {
        b.unionR