function getMarker(node, markerType, parentNode) {
  var id = referenceToId(svgAttr(node, markerType));

  return domQuery('marker#' + id, parentNode || document);
}
Get marker of given type referenced by node.

@param {HTMLElement} node
@param {string} markerType
@param {HTMLElement} [parentNode]

@param {HTMLElement}