systemd WantedBy for X login

[Aidan Gauland <aidalgol@fastmail.net>]

I have a bunch of systemd user services for headless daemons that I want
to run in both console and graphical logins, which all work fine, but I
have one that I only want to start for graphical logins, and that works
when I start it manually, but it is not being started when I log in,
even though I have the WantedBy field set to graphical.target.  I am
using lightdm and i3.  The other services' WantedBy field is set to
default.target.  What is the correct way to specify that a service
should be started only for X logins?

Regards,
Aidan Gauland