There are two keysyms for grave,
dead_grave, by default
dead_grave is set to the ` key. xmodmap can be used to change it, and should be in Ubuntu by default (with arch install xorg-xmodmap).
To change it from the default
xmodmap -e "keycode 49 = grave notsign grave notsign brokenbar notsign brokenbar notsign"
The rest of the values are various modifiers, e.g. the second one is shift.
If you want this to be permanent, create
~/.Xmodmap, and then add just:
keycode 49 = grave notsign grave notsign brokenbar notsign brokenbar notsign
If there are any other dead keys you're having trouble with, run:
xmodmap -pke | grep dead
And it should give you a list of dead keys. Alternatively you could run:
xmodmap -pke >> ~/.Xmodmap
You will then have a full list of keyboard keys and their sym values in
.Xmodmap, which makes it easy to change your dead keys and any other keys to whatever custom layout you like.