function getLineLength(el) {
  const x1 = el.getAttribute('x1');
  const x2 = el.getAttribute('x2');
  const y1 = el.getAttribute('y1');
  const y2 = el.getAttribute('y2');
  return Math.sqrt((x2 - x1) ** 2 + (y2 - y1) ** 2);
}
Returns the `<line>` length.
@param {SVGLineElement} el target element
@returns {number} the element length