Making the contour lines I have encountered a bug for the using of suffix 'a' in order to indicate Angstrom instead of Bohr. Here you are the sequence used after the corresponding topological analysis from the main menu:
4/1/2/grid selection/1(xy plane)/for z I used 0.0a and the units of the generated graph are always Bohr instead of Angstrom. Doesn't matter because you can use option -8 that works correctly, but this problem seems not to appear for the other plane directions. Just a little detail. Thanks again!