def _el_orb_tuple(string):
    """Parse the element and orbital argument strings.

    The presence of an element without any orbitals means that we want to plot
    all of its orbitals.

    Args:
        string (`str`): The selected elements and orbitals in in the form:
            `"Sn.s.p,O"`.

    Returns:
        A list of tuples specifying which elements/orbitals to plot. The output
        for the above example would be:

            `[('Sn', ('s', 'p')), 'O']`
    """
    el_orbs = []
    for split in string.split(','):
        splits = split.split('.')
        el = splits[0]
        if len(splits) == 1:
            el_orbs.append(el)
        else:
            el_orbs.append((el, tuple(splits[1:])))
    return el_orbs