The following problem appears when using Grafana 6.7 only (verified with 6.7.2 and 6.7.3). It doesn't appear on 6.6.x where Helm works perfectly fine (tested with 6.6.2).
It is essential to mention that the Docker images for Helm are based on 6.7.2, which is another reason for having this fixed soon.
When you add a graph panel, choose the PM data source, and click on Node, you'll see something like the attached screenshot and stays like that forever. The same thing happens when trying to choose a resource (after picking a node).
However, I found that if I interact with the dialog, the list renders. For instance, click on the panel, interact with the search/query field, a combination of both, etc. It looks like if something forgot to tell Grafana that the retrieval of the data is done.
The current workaround is downgrade Grafana to 6.6.x, and if you're using Docker, use Grafana's official image instead of the Helm image.