def default_unit_label(axis, unit):
    """Set default label for an axis from a `~astropy.units.Unit`

    If the axis already has a label, this function does nothing.

    Parameters
    ----------
    axis : `~matplotlib.axis.Axis`
        the axis to manipulate

    unit : `~astropy.units.Unit`
        the unit to use for the label

    Returns
    -------
    text : `str`, `None`
        the text for the new label, if set, otherwise `None`
    """
    if not axis.isDefault_label:
        return
    label = axis.set_label_text(unit.to_string('latex_inline_dimensional'))
    axis.isDefault_label = True
    return label.get_text()
