def external_fig_and_ax_3d(fig=None, ax=None):
    """
    *Existing Matplotlib figure and 3D axes*

    Parameters:
        fig: A Matplotlib Figure object. Contains one or several Axes objects.
            If in doubt, look here: https://matplotlib.org/faq/usage_faq.html
        ax: A Matplotlib 3D Axes object to draw to. Done via the plotting methods it provides.
    """
