I faced with this problem many times (along ~ last week), and finally i can reproduce it.

1. Go to Monitoring -> Graphs
2. Select some host and it's some graph
3. Select the another host
4. Voila

When i select another host on step #3 then "hostid" in the URL changes but "graphid" stays as it was.

And on top of the page we see:
Undefined index: graphid [api/classes/class.cgraph.php:382]
Undefined index: graphid [api/classes/class.cgraph.php:382]
Undefined index: graphid [api/classes/class.cgraph.php:388]
Undefined index: graphid [api/classes/class.cgraph.php:389]
Undefined index: graphid [api/classes/class.cgraph.php:424]

Link on the picture is:
/zabbix/charts.php?sid=4ba4e1d7fd7e416b&form_refresh=1&fullscreen=0&groupid=0&hostid=10017&graphid=563
Note - graph with the ID=563 not related to host with ID=10017

This link works without errors:
zabbix/charts.php?sid=4ba4e1d7fd7e416b&form_refresh=1&fullscreen=0&groupid=0&hostid=10049&graphid=563
Note - here is another (proper) "hostid".