Hello Francesco,
Your Chimera is super-old, but I don't think that makes a difference to this particular feature especially if you have used it before.  Still, I tested the below in the last production release, 1.17.3.

If you mean showing a dashed line, it does not do that in Chimera for centroids.  It just reports the distance in the axes/planes/centroids dialog and/or in the status line and Reply Log, depending on which measurement method you used.  Commands example:

open 2gbp
define centroid ligand
define centroid :280
distance c1 c2

Distance reported instatus line as 28.56

Or if you just choose both rows for those centroids in the axes/planes/centroids dialog, or Ctrl-click one and Shift-Ctrl-click the other one, it will report distance 28.6 near the bottom of the axes/planes/centroid dialog.

The identifiers (in this case "c1" and "c2") are automatically shown in the axes/planes/centroids dialog when you define the centroids.

I don't know which manual page you are viewing.  Maybe you are viewing the ChimeraX help, which is different than in Chimera.

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Dec 16, 2023, at 7:36 AM, Francesco Pietra via Chimera-users <chimera-users@cgl.ucsf.edu> wrote:
>
> Hello
>
> with candidate version 1.15, linux64, X11
> I want to measure the distance between the centroids of two aromatic stacking rings. The usual way of measuring distances does not work (clicking as needed has no effect), contrary to what the manual says. And I should have done successfully that measurement in the past.
>
> Also between one of the centroids and a regular atom: same issue.
>
> Distance between regular atoms works correctly.
>
> thanks for advice
> francesco pietra
>