What do the symbols in the graphic of the dynamic analysis mean?
The symbols in the DYNAM graphic indicate twisting members.
If there wasn't such an indication, it wouldn't be possible to recognize any torsion, as the members are related to the centroidal axis in the graphic. This is the reason why a short vertical line with small end circles was chosen for the graphical representation.
In the animation mode, you will see that these symbols rotate, thus representing torsions.