code: def _ar_matrix(self):
          
          Y = np.array(self.data[self.max_lag:self.data.shape[0]])
          X = self.data[(self.max_lag-1):-1]
  
          if self.ar != 0:
              for i in range(1, self.ar):
                  X = np.vstack((X,self.data[(self.max_lag-i-1):-i-1]))
  
          return X
docs: Creates Autoregressive matrix
  
          Returns
          ----------
          X : np.ndarray
              Autoregressive Matrix