Hello! Thanks for posting!
What you see in Navigraph Charts is the FIR boundary - not the ATC sectors. ATC can decide to divide their airspace differently based on what makes sense for operations, especially close to borders.
Skyvector and the FAA Sectionals seem to agree with us on where the FIR border should be placed:
Once we integrate online networks in VATSIM/ IVAO Integration - Active ATC with respective frequencies and traffic, you should be able to more easily tell which ATC sector you are in!
Kind Regards,
Malte