On further investigation, it is not needed explicitly because systemctl is smart enough to launch the dgraph-zero service because the dgraph service depends on it as shown here?
Here are the docs on systemctl which are applicable: https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Requires=
The important bit being:
If this unit gets activated, the units listed (under
Requires=) will be activated as well.