code: def get_xy(self, yidx, xidx=0):
          
          assert isinstance(xidx, int)
          if isinstance(yidx, int):
              yidx = [yidx]
  
          t_vars = self.concat_t_vars()
  
          xdata = t_vars[:, xidx]
          ydata = t_vars[:, yidx]
  
          return xdata.tolist(), ydata.transpose().tolist()
docs: Return stored data for the given indices for plot
  
          :param yidx: the indices of the y-axis variables(1-indexing)
          :param xidx: the index of the x-axis variables
          :return: None