code: func (pwl PieceWiseLinear) Valid() bool {
  	var lastX uint64
  	for _, i := range pwl {
  		if i.X < lastX {
  			return false
  		}
  		lastX = i.X
  	}
  	return true
  }
docs: // Valid returns true if the X coordinates of the curve points are non-strictly monotonic