How to use the astroplan.exceptions.PlotWarning function in astroplan

if ax is None:
        ax = plt.gca()
    if style_kwargs is None:
        style_kwargs = {}
    style_kwargs = dict(style_kwargs)
    style_kwargs.setdefault('linestyle', '-')
    style_kwargs.setdefault('fmt', '-')

    # Populate time window if needed.
    time = Time(time)
    if time.isscalar:
        time = time + np.linspace(-12, 12, 100)*u.hour
    elif len(time) == 1:
        warnings.warn('You used a Time array of length 1.  You probably meant '
                      'to use a scalar. (Or maybe a list with length > 1?).',
                      PlotWarning)

    # Calculate parallactic angle.
    p_angle = observer.parallactic_angle(time, target)

    # Some checks & info for labels.
    assert len(time) == len(p_angle)

    if not hasattr(target, 'name'):
        target_name = ''
    else:
        target_name = target.name
    style_kwargs.setdefault('label', target_name)

    # Plot data.
    ax.plot_date(time.plot_date, p_angle, **style_kwargs)

class TargetNeverUpWarning(AstroplanWarning):
    """Target never rises above horizon"""
    pass


class OldEarthOrientationDataWarning(AstroplanWarning):
    """Using old Earth rotation data from IERS"""
    pass


class PlotWarning(AstroplanWarning):
    """Warnings dealing with the plotting aspects of astroplan"""
    pass


class PlotBelowHorizonWarning(PlotWarning):
    """Warning for when something is hidden on a plot because it's below the horizon"""
    pass

style_kwargs.setdefault('fmt', '-')

    if hasattr(time, 'utcoffset') and use_local_tz:
        tzoffset = time.utcoffset()
        tzname = time.tzname()
    else:
        tzoffset = 0
        tzname = 'UTC'
    # Populate time window if needed.
    time = Time(time)
    if time.isscalar:
        time = time + np.linspace(-12, 12, 100)*u.hour
    elif len(time) == 1:
        warnings.warn('You used a Time array of length 1.  You probably meant '
                      'to use a scalar. (Or maybe a list with length > 1?).',
                      PlotWarning)

    if not isinstance(targets, Sequence):
        targets = [targets]

    for target in targets:
        # Calculate airmass
        airmass = observer.altaz(time + tzoffset, target).secz
        # Mask out nonsense airmasses
        masked_airmass = np.ma.array(airmass, mask=airmass < 1)

        # Some checks & info for labels.
        try:
            target_name = target.name
        except AttributeError:
            target_name = ''