Thanks Eric, I got it to work now. To summarize the code to get the axis information from an atom spec string looks like this. Please let me know if there is a better or simpler way to do it.
#-----------------------------------
import StructMeasure
atomspec = '#0:14-37.A@CA'
atoms = chimera.selection.OSLSelection(atomspec).vertices()
xyzs = chimera.numpyArrayFromAtoms(atoms)
myaxis = StructMeasure.axis(xyzs)
print myaxis#-------------------------------------
Note that I had to import StructMeasure.
Best
H.
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