Authenticate users using reverse proxy + allow read-only access to view certain graphs

Dear all,
I have set up Grafana using this guide behind Apache and that works as expected.
Now I would like to be able to embed some graphs to external websites in read-only mode without authentication. I have copied the URL which is like this one:

but unfortunately it does not render as in this flow the header X-WEBAUTH-USER is missing which results Grafana to fall back to Basic authentication. I have configuration Apache to add the header for above URL

RequestHeader set X-WEBAUTH-USER guest

and it works fine for 1st (top) request, but fails for all subrequests.

I tried the setup when [auth.anonymous] is enabled, but it results that any client may view any Grafana dashboard and any graph, however my intention was that read-only access is limited to a specific set of graphs (e.g. could be configured in Apache) and other access should be authenticated.

I also looked into permanent authentication token option, but it turned out to be equivalent to above: the 1st request is authenticated, but the following ones are not.

Any idea about how to correctly implement? Thanks in advance.