Yes, Graphite integration is still available in OCUM 7.3 (that includes OPM) :
Configuring a connection from a Unified Manager server to an external data provider
From there you just have to use Grafana/Graphite integration that is not dependant on NetApp options. Keep in mind that you will have to build your own Dashboards, as there is no pre-built option that I know of.
That's the other great thnig about Harvest, it comes with a plethora of pre-built dashboard but they are only compatible with the set of metrics collected by Harvest (which are much more than the set provided by just OPM)
Hope it helps.