code: function getMarker(node, markerType, parentNode) {
    var id = referenceToId(svgAttr(node, markerType));
  
    return domQuery('marker#' + id, parentNode || document);
  }
docs: helpers ////////// 
  Get marker of given type referenced by node.
  
  @param {Node} node
  @param {string} markerType
  @param {Node} [parentNode]
  
  @param {Node}