def concat_t_vars_np(self, vars_idx=None):
        """
        Concatenate `self.np_t` with `self.np_vars` and return a single matrix.
        The first column corresponds to time, and the rest of the matrix is the variables.

        Returns
        -------
        np.array : concatenated matrix

        """
        selected_np_vars = self.np_vars
        if vars_idx is not None:
            selected_np_vars = self.np_vars[:, vars_idx]

        return np.concatenate([self.np_t[:self.np_nrows].reshape((-1, 1)),
                               selected_np_vars[:self.np_nrows, :]], axis=1)