Absolutely many of administrators worked with "cacti". in Cacti we can "sum" 2 or more graphs in one display.for example you can sum graph1 and graph2 on one display.
The only way I found to do this in zabbix is ,first create one new item as type calculated for item_1 and item_2.and then create graph for that new item!
According to reported problem in " https://support.zabbix.com/browse/ZBXNEXT-615 " topic this way is not very good for creating sum graph.
So the better way is sum graphs as same as cacti does.
If anybody see this feature useful please vote this requested feature.
Thanks in advance