def METHOD_NAME(ax=None):
    """Helper function for when a single plot axis is needed.
    Parameters
    ----------
    ax : AxesSubplot, optional
        If given, this subplot is used to plot in instead of a new figure being
        created.
    Returns
    -------
    fig : Figure
        If `ax` is None, the created figure.  Otherwise the figure to which
        `ax` is connected.
    ax : AxesSubplot
        The created axis if `ax` is None, otherwise the axis that was passed
        in.
    Notes
    -----
    This function imports `matplotlib.pyplot`, which should only be done to
    create (a) figure(s) with ``plt.figure``.  All other functionality exposed
    by the pyplot module can and should be imported directly from its
    Matplotlib module.
    See Also
    --------
    create_mpl_fig
    Examples
    --------
    A plotting function has a keyword ``ax=None``.  Then calls:
    >>> from statsmodels.graphics import utils
    >>> fig, ax = utils.create_mpl_ax(ax)
    """
    if ax is None:
        plt = _import_mpl()
        fig = plt.figure()
        ax = fig.add_subplot(111)
    else:
        fig = ax.figure
    return fig, ax