'Plots the impulse response, spectral_density, autocovariance,
and one realization of the process.'

def quad_plot(self):

    (num_rows, num_cols) = (2, 2)
    (fig, axes) = plt.subplots(num_rows, num_cols, figsize=(12, 8))
    plt.subplots_adjust(hspace=0.4)
    plot_functions = [self.plot_impulse_response, self.plot_spectral_density, self.plot_autocovariance, self.plot_simulation]
    for (plot_func, ax) in zip(plot_functions, axes.flatten()):
        plot_func(ax, show=False)
    plt.show()
