Hello lyhour,
If you have the coordinate data in the external file, you should call chunk.importReference() command.
You can also use ignore_labels=True argument for this call, in this case the labels will be automatically re-assigned, based on the information in the input text file(label, X-coord, Y-coord, Z-coord), like you have requested in another thread:
#N,X,Y,Z
1,0.0,10.0,0.0
2,0.0,20.0,0.0
....
10.10.0,10.0,0.0
But since there's some symmetry in the marker distribution, the corner, from which the naming will be started, may be random.