def _title(c, index):
    """Generic image title if not provided.

    Parameters
    -----------
    c: pyvims.VIMS
        Cube to plot.
    index: int, float, str or tuple
        VIMS band or wavelength to plot.

    Returns
    -------
    str
        Default title.

    """
    if isinstance(index, int):
        return f'{c} on band {index}'

    if isinstance(index, float):
        return f'{c} at {index:.2f} µm'

    if isinstance(index, str):
        if 'um' in index:
            return f'{c} | {index.replace("um", " µm")}'
        return f'{c} | {index.title()}'

    if isinstance(index, tuple):
        if isinstance(index[0], float):
            return f'{c} at ({index[0]:.2f}, {index[1]:.2f}, {index[2]:.2f}) µm'

        else:
            return f'{c} on bands {index}'

    return None