def plot_before_after_filter(signal, sr, band_begin, band_end, order=1, x_lim=[], y_lim=[],
                             orientation="hor", show_plot=False, file_name=None):
    """
    -----
    Brief
    -----
    The use of the current function is very useful for comparing two power spectrum's (before and
    after filtering the signal).
    This function invokes "plot_informational_band" in order to get the power spectrum before
    applying the signal to the lowpass filter.

    -----------
    Description
    -----------
    The FFT Power Spectrum, of an input signal, can be generated through plotfft function of
    novainstrumentation package (or periogram function of scipy package).
    The x axis (freqs) represents the frequency components of the signal, after decomposition was
    achieved by applying the Fourier Transform. The y axis (power) defines the relative weight of
    each frequency component (sinusoidal function) in the process of reconstructing the signal by
    re-summing of decomposition components.

    It is presented a 1x2 gridplot for compaing the differences in frequency composition of the
    signal under analysis (before and after filtering).

    Additionally, it is also graphically presented a rectangular box showing which are the frequency
    components with relevant information for studying our input physiological signal.

    Applied in the Notebook "Digital Filtering - A Fundamental Pre-Processing Step".

    ----------
    Parameters
    ----------
    signal : list
        List containing the acquired signal samples.

    sr : int
        Sampling rate.

    band_begin : float
        Lower frequency inside the signal informational band.

    band_end : float
        Higher frequency inside the signal informational band.

    order : int
        Filter order.

    x_lim : list
        A list with length equal to 2, defining the first and last x value that should be presented.

    y_lim : list
        A list with length equal to 2, defining the first and last y value that should be presented.

    orientation : str
        If "hor" then the generated figures will be joined together in an horizontal gridplot.
        When "vert" the gridplot will be a vertical grid and when "same" the plots are generated at
        the same figure.

    show_plot : bool
        If True then the generated figure/plot will be shown to the user.

    file_name : str
        Path containing the destination folder where the Bokeh figure will be stored.

    Returns
    -------
    out : list
        List of Bokeh figures that compose the generated gridplot.
    """

    # Generation of the HTML file where the plot will be stored.
    #file_name = _generate_bokeh_file(file_name)

    # Generation of FFT power spectrum accordingly to the filter order.
    for i in range(0, order + 1):
        # Initialisation and appending of data to the figures list.
        if i == 0:
            # Power spectrum
            freqs_after, power_after = plotfft(signal, sr)
            figure_after = plot_informational_band(freqs_after, power_after, signal, sr,
                                                   band_begin, band_end,
                                                   legend="Signal Power Spectrum", x_lim=x_lim,
                                                   y_lim=y_lim)
            # List that store the figure handler
            list_figures = [[figure_after]]
        else:
            filter_signal = lowpass(signal, f=band_end, order=i, fs=sr)

            # Power spectrum
            freqs_after, power_after = plotfft(filter_signal, sr)

            if orientation != "same":
                figure_after = plot_informational_band(freqs_after, power_after, filter_signal, sr,
                                                       band_begin, band_end,
                                                       legend="Filtered FFT (Order " + str(i) + ")",
                                                       x_lim=x_lim, y_lim=y_lim)
                # Append data accordingly to the desired direction of representation.
                if orientation == "hor":
                    # Append to the figure list the power spectrum of the signal after filtering.
                    list_figures[-1].append(figure_after)
                elif orientation == "vert":
                    list_figures.append([figure_after])
            else:
                list_figures[-1][0].line(freqs_after, power_after, legend="Filtered FFT (Order " + str(i) + ")",
                                         **opensignals_kwargs("line"))

    # Show gridplot.
    grid_plot_1 = gridplot(list_figures, **opensignals_kwargs("gridplot"))

    if show_plot is True:
        show(grid_plot_1)

    return list_figures