function getEllipseLength(el) {
  const rx = el.getAttribute('rx');
  const ry = el.getAttribute('ry');
  const len = 2 * rx;
  const wid = 2 * ry;
  return ((Math.sqrt(0.5 * ((len * len) + (wid * wid)))) * (Math.PI * 2)) / 2;
}
Returns the `<ellipse>` length.
@param {SVGEllipseElement} el target element
@returns {number} the element length