def register_axis(key=None):
    """Returns a decorator registering an axis class in the axis type registry.

    If no key is provided, the class name is used as a key. A key is provided
    for each core bqplot axis so that the frontend can use this key regardless
    of the kernel language.
    """
    def wrap(axis):
        name = key if key is not None else axis.__module__ + axis.__name__
        BaseAxis.axis_types[name] = axis
        return axis
    return wrap