To type Latin Macrons āēīōū

in X generally, you need to use the Compose key, most keyboards don't have one, so Compose is usually done with shift+Alt Gr,

Compose - a and Compose a - will both produce an ā

but it's more convenient to make the right Alt key (AltGr) be the compose key

to try it in a terminal
setxkbmap -option "compose:ralt"

to make it the system default:
sudo dpkg-reconfigure keyboard-configuration

you can also get it to work just in your desktop environment with e.g. in xfce, Applications/Settings/Keyboard/Layout