Blank iframe-s in Home Assistant

Faced a problem trying to use Grafana in Home Assistant (not HASS.io).
Installed it on Rasp 3B according to manual. Graphs are successfully rendering and can be shared.
I may even open them with direct link. But after integrating them to Home Assistant with iframe I can only see blank frame without data. What do I miss? Does Grafana need to be additionally configured somewhere in HA?