def _add_sphere(ax):
    """ _add_sphere(ax)

    Add a wireframe unit sphere onto matplotlib 3D axes

    Args:
        ax - matplotlib 3D axes object

    Returns:
        updated matplotlib 3D axes
    """
    (u, v) = np.mgrid[0:2 * np.pi:20j, 0:np.pi:10j]
    x = np.cos(u) * np.sin(v)
    y = np.sin(u) * np.sin(v)
    z = np.cos(v)
    ax.plot_wireframe(x, y, z, color='grey', linewidth=0.2)
    return ax