import c4d
from c4d.gui import MessageDialog

def main():
    if not op:
        # op is None, no selected object or multiple ones
        MessageDialog("You must select exactly one object.")
        return
    if not isinstance(op, c4d.PointObject):
        # selected object is not a PointObject, we cannot center the axis
        MessageDialog("You must select either a spline or polygon object")
        return

    points = op.GetAllPoints()

    min = c4d.Vector()
    max = c4d.Vector()

    op_position = op.GetAbsPos()
    for p in points:
        p = p + op_position
        if p.x < min.x:
            min.x = p.x
        if p.x > max.x:
            max.x = p.x

        if p.y < min.y:
            min.y = p.y
        if p.y > max.y:
            max.y = p.y

        if p.z < min.z:
            min.z = p.z
        if p.z > max.z:
            max.z = p.z

    center = (min + max) / 2.
    print min, max, center
    # ...

    for index, p in enumerate(points):
        p = p - center
        points[index] = p
    op.SetAllPoints(points)
    """
    op.SetAbsPos(op_position - center)"""

    op.Message(c4d.MSG_UPDATE)
    c4d.EventAdd()

main()