code: function(elm1, elm2) {
  
      var myLink = new erd.Line({
          markup: [
              '<path class="connection" stroke="black" d="M 0 0 0 0"/>',
              '<path class="connection-wrap" d="M 0 0 0 0"/>',
              '<g class="labels"/>',
              '<g class="marker-vertices"/>',
              '<g class="marker-arrowheads"/>'
          ].join(''),
          source: { id: elm1.id },
          target: { id: elm2.id }
      });
  
      return myLink.addTo(graph);
  }
docs: Helpers