function setMarker (el, val) {
      const markerName = 'marker-start';
      const marker = getLinked(el, markerName);
      if (marker) { marker.remove(); }
      el.removeAttribute(markerName);
      if (val === 'nomarker') {
        svgCanvas.call('changed', [ el ]);
        return;
      }
      // Set marker on element
      const id = 'placemark_marker_' + el.id;
      addMarker(id, val);
      el.setAttribute(markerName, 'url(#' + id + ')');
      svgCanvas.call('changed', [ el ]);
    }
@param {Element} el
@param {string} val
@returns {void}