The Disk Usage Analyzer defaults to scanning your home folder. You can select
"Scan Filesystem", which will begin the scan at your root. (But the scan will
include your home folder, too.)

That you have less than 1 gb left from a 50 gb root partition makes me think
that there's something pathological going on - perhaps an exploding error log
file?

The root will have a name like sda1 or hda3, with aliases like disk or
disk-1. Not very intuitive I agree! You can find the name from the command
line by typing df -h which gives you the 'Disk (space) Free (in a) human
(readable form)'.
All in lower case.

That's a lot of root you have used up. Could it be that you have stored a
bunch of data on your root directory which could perhaps be more
appropriately located in your home directory?