Lay out nodes on a circle and add node index.

def circular_layout(nodes):
    """
    Lay out nodes on a circle and add node index.
    """
    N = len(nodes)
    if not N:
        return ([], [], [])
    circ = np.pi/N*np.arange(N)*2
    x = np.cos(circ)
    y = np.sin(circ)
    return (x, y, nodes)