 (let i = 0; i < ndata.length; i++) {
          const child = ndata[i];
          const parentkey = model.getParentKeyForNodeData(child);
          const parent = model.findNodeDataForKey(parentkey);
          if (parent !== null) {
            // if there is a parent, there should be an edge
            // now find corresponding vertexes
            const f = dataVertexMap.get(parent);
            const t = dataVertexMap.get(child);
            if (f === null || t === null) continue; // skip
            //