How to use the dygraphs/src/dygraph.prototype function in dygraphs

if (p.yval === null) {
    return false
  } // missing point
  if (p.x === null || p.x === undefined) {
    return false
  }
  if (p.y === null || p.y === undefined) {
    return false
  }
  if (isNaN(p.x) || (!opt_allowNaNY && isNaN(p.y))) {
    return false
  }
  return true
}

Dygraph.prototype.findClosestPoint = function(domX, domY) {
  if (Dygraph.VERSION !== '2.0.0') {
    console.error(
      `Dygraph version changed to ${Dygraph.VERSION} - re-copy findClosestPoint`
    )
  }
  let minXDist = Infinity
  let minYDist = Infinity
  let xdist, ydist, dx, dy, point, closestPoint, closestSeries, closestRow
  for (let setIdx = this.layout_.points.length - 1; setIdx >= 0; --setIdx) {
    const points = this.layout_.points[setIdx]
    for (let i = 0; i < points.length; ++i) {
      point = points[i]
      if (!isValidPoint(point, null)) {
        continue
      }