docs: def decay(ax, p0, pf, A, n, format=None, **kwds):
      r"""Draw a spontaneous decay as a wavy line."""
      if format is None: format = 'k-'
      T = sqrt((p0[0]-pf[0])**2+(p0[1]-pf[1])**2)
      alpha = atan2(pf[1]-p0[1], pf[0]-p0[0])
  
      x = [i*T/400.0 for i in range(401)]
      y = [A*sin(xi * 2*pi*n/T) for xi in x]
  
      cur_list = [(x, y)]
      cur_list = rotate_and_traslate(cur_list, alpha, p0)
  
      for curi in cur_list:
          ax.plot(curi[0], curi[1], format, **kwds)