It took me a while and some hacking until I figured out how to enable 1366×768 screen resolution in ubuntu 16-04 for my secondary display, a Fujitsu-Siemens TV from pre-HD era, means: HD ready but not providing the full resolution, just 1366×768.
Anyway, here is the magic:
cvt 1366 768 60
xrandr –newmode "1368x768_60.00" 85.25 1368 1440 1576 1784 768 771 781 798 -hsync +vsync
xrandr –addmode VGA-0 "1368x768_60.00"
xrandr –output VGA-0 –mode "1368x768_60.00"
Nevertheless, I must say, when I had Fedora 25 installed on my computer beforehand it offered 1368×768 screen resolution out of the box, ubuntu didn’t. Well, that’s what makes linux so exciting for many: the need to tinker and learn interesting technical, sometimes magical secrets.
Thanks to this internet articles I finally solved my problem: