def build_secondary_axis(axis, on='x', **kwargs):
    """
    Build twin secondary axis.

    :param axis: Matplotlib axis instance.
    :type axis: :class:`matplotlib.axes.Axes`
    :param on: Name of axis to build secondary axis (x, y).
    :type on: str, default: x
    """
    methods = {'x': 'twinx', 'y': 'twiny'}
    method = getattr(axis, methods[on])
    return method()
