import GraphmlModdle from 'graphml-moddle';

var moddle = new GraphmlModdle();

var modelXML = ...;

moddle.fromXML(modelXML, function(err, graphml) {

  // read graphs
  graphml.rootElements; // [ { $type: 'graphml:Graph', ... }, ... ]

  // read extensions
  graphml.extensions; // [ { $type: 'graphml:Key', ... }, ... ]

  // add a new graph
  const newGraph = moddle.create('graphml:Graph', {
    id: 'myGraph'
  });

  graphml.rootElements.push(newGraph);

  moddle.toXML(graphml, function(err, updatedXML) {

    // updatedXML contains new graph

  });

});