Global zone server count graph missing

[avijc]

At the bottom of each continent page e.g. https://www.ntppool.org/zone/europe there is a graph "Server counts for ".
The Global page https://www.ntppool.org/zone/@ has only "Error: Missing required zone attribute" where the graph should show up.

[avijc]

"All Pool Servers" graph at https://www.ntppool.org/zone works, though.