getElem_ (id) {
    if (this.svgElem_.querySelector) {
      // querySelector lookup
      return this.svgElem_.querySelector('#' + id)
    }
    // jQuery lookup: twice as slow as xpath in FF
    return this.svgElem_.querySelector('[id=' + id + ']')
  }
@param {string} id Element ID to retrieve
@returns {Element} SVG element within the root SVGSVGElement