It's a small thing, but nothing in the documentation really suggests to me that the graph won't appear until the downstream job is actually built as a downstream job triggered by the upstream.

I installed the plugin, saw that it should just work, and couldn't find the graph. Spent an embarrassingly long time double-checking my configuration on the downstream job and rebuilding it, looking for bug reports, rebooting my Jenkins server, etc, before noticing the dependency on "Downstream Build Cache" and guessing that maybe that meant it was going to cache builds triggered as downstreams, and wouldn't have any data until then.

Alternatively, if that could be enhanced so that the plugin could handle upstream as well it would be great - but I gather the intention of this plugin is not to show the general relationships of a job so much as the specific build relationship for a build?