code: def _sort_tau_by_y(self, y):
          
          # first column is the variable of interest
          tau_y = self.tau_matrix[:, y]
          tau_y[y] = np.NaN
  
          temp = np.empty([self.n_nodes, 3])
          temp[:, 0] = np.arange(self.n_nodes)
          temp[:, 1] = tau_y
          temp[:, 2] = abs(tau_y)
          temp[np.isnan(temp)] = -10
          tau_sorted = temp[temp[:, 2].argsort()[::-1]]
  
          return tau_sorted
docs: Sort tau matrix by dependece with variable y.
  
          Args:
              :param y: index of variable of intrest
              :type y: int