code: def _apply_rotation(self,dx,dy):
          
          if self.anisotropy == 1.0:
              return dx,dy
          rcoefs = self.rotation_coefs
          dxx = (dx * rcoefs[0]) +\
               (dy * rcoefs[1])
          dyy = ((dx * rcoefs[2]) +\
               (dy * rcoefs[3])) *\
               self.anisotropy
          return dxx,dyy
docs: private method to rotate points
          according to Vario2d.bearing and Vario2d.anisotropy
  
          Parameters
          ----------
          dx : (float or numpy.ndarray)
              x-coordinates to rotate
          dy : (float or numpy.ndarray)
              y-coordinates to rotate
  
          Returns
          -------
              dxx : (float or numpy.ndarray)
                  rotated x-coordinates
              dyy : (float or numpy.ndarray)
                  rotated y-coordinates