For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:berny:branches:openSUSE:Leap:15.2:Update/15.5/home:berny:branches:openSUSE:Leap:15.2:Update.repo
zypper refresh
zypper install graphviz
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:berny:branches:openSUSE:Leap:15.2:Update/15.4/home:berny:branches:openSUSE:Leap:15.2:Update.repo
zypper refresh
zypper install graphviz
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:berny:branches:openSUSE:Leap:15.2:Update/15.3/home:berny:branches:openSUSE:Leap:15.2:Update.repo
zypper refresh
zypper install graphviz