The following scenario worked for me:

1. Create a log file /tmp/simple-graphs.log with two lines: "error" and "warning".
2. Create item "a simple graph test item" that observes this log file with key "log[/tmp/simple-graphs.log]".
3. Create trigger "a simple graph test trigger - 1" with expression "

{Zabbix Server:log[/tmp/simple-graph.log].iregexp(error)}

#0".
4. Clone that trigger to create another trigger "a simple graph test trigger - 2" with "

{Zabbix Server:log[/tmp/simple-graph.log].regexp(warning)}

#0".
5. Wait until events appear in "Monitoring" -> "Events".

If we click on description for "a simple graph test trigger - 1", the "Simple graphs" menu is empty (zero.png).
If, however, we click on description for "a simple graph test trigger -2", the "Simple graphs" menu has two identical items (multiple.png).