Hello,
In linux, folders with a dot "." as a prefix have a "hidden attribute", therefore ".qgis" folder located in home directory
is a hidden one.
You can view hidden folders pressing "Ctrl+H" in nautilus (the default manager for files in Ubuntu) and they will appear with a transparent layout. After that you can install your plugin
following this answer in S.O.
BTW, Artelia has a really great plugin for displaying results in Qgis, It does need a few dependencies (matplotlib, networkx, numpy/scipy, shapely if I recall correctly) which you could install using:
sudo apt install python-matplotlib python-numpy python-shapely python-scipy python-networkx
Check their
github wiki page for further information about installation and usage.
Regards,
José Díaz.