def parse_range_specification(spec):
    """Parses a range specification used as arguments for some options
    in ``qplot``. Range specifications contain two numbers separated
    by a colon (``:``). Either of the numbers may be replaced by an
    underscore (``_``) or an empty string meaning 'automatic'."""
    return [
        None if value == "_" or value == "" else float(value)
        for value in spec.split(":", 1)
    ]