Please make sure that you call the charts/graphs not with an HTTPS prefix. HTTPS brings additional overhead to the server, which is not necessary, as you dont transfer sensible information when requesting display of data.
When you login to the admin interface of Meteoplug, I do strongly recommend to use HTTPS, as you hand over credentials that others might not be able to sniff from the raw TCP/IP packets.
Rule of thumb:
- Charting, Graphs, etc -> HTTP
- Admin interface -> HTTPS