The script below reads the images in a saved MD trajectory and prints the coordinates of a particular atom (identified by the index i) in each image. Pretty easy to expand this script to also store the coordinates in a list for plotting purposes.
trajectory = nlread('MD_trajectory.hdf5', MDTrajectory)[-1]
images = trajectory.coordinates()
i = 0
for image in images:
position = image[i]
print position
That should have worked in the 2017 version. For ATK-2016, you should adjust the script as follows.
trajectory=nlread('trajectory.nc',MDTrajectory)[-1]
images=trajectory.coordinates()
for image in range(len(images)):
elements=trajectory.image(image).elements()
for n in range(len(elements)):
element=elements[n].symbol()
if element=='H':
position=images[image][n]
nlprint(position)