docs: def findXAt(xArr, yArr, yVal, index=0, s=0.0):
      """
      index: position of root (return index=0 by default)
  
      return all x values where y would be equal to given yVal
      if arrays are spline interpolated
      """
      if xArr[1] < xArr[0]:
          # numbers must be in ascending order, otherwise method crashes...
          xArr = xArr[::-1]
          yArr = yArr[::-1]
  
      yArr = yArr - yVal
      if len(yArr) < 5:
          xn = np.linspace(xArr[0], xArr[-1], 5)
          yArr = np.interp(xn, xArr, yArr)
          xArr = xn
      f = interpolate.UnivariateSpline(xArr, yArr, s=s)
      return f.roots()[index]