def set_axis(ax, letter=None):
    """
    Formats the plot's caption.

    Parameters
    ----------
    ax: Axes object.
    letter: string
        Caption of the plot.
        Default: None.

    Returns
    -------
    ax: modyfied Axes object.
    """
    ax.text(
        -0.15,
        1.05,
        letter,
        fontsize=18,
        weight='bold',
        transform=ax.transAxes)
    return ax