There is an opportunity in 3.4 to create a global and a host/template macro for working time, as shown in this article: https://habrahabr.ru/company/zabbix/blog/336084/. Great!
But in a graph I can only see the global working time regardless the host or template level macro. I understand there is a problem with several items which belongs to hosts with different working time on the same graph, but may be in this case just use working time of the first item in list? It's a simple algorythm and it allows a user to easy choose desired working time interval for graph.