Problem description:
If a trigger uses function 'change', the problem raised by this trigger will not be shown on the classic graph that displays the item this trigger is based on. But if the function used by the trigger is changed, even while the trigger is in status problem, the trigger will be shown on the graph.

Steps to reproduce:

1. Create a host
2. Create a numeric(unsigned) item with units = 'uptime', update interval = '1s' and key = 'system.uptime'
3. Create a trigger with the following expression: {Test:system.uptime.change(0)}

   >10m

4. Create a graph. Set the 'Show triggers' option and select the previously created item in 'items' field.
5. Restart zabbix server
6. Open Monitoring => Problems view and make sure that the trigger has raised a problem
7. Open the previously created graph anywhere in frontend
   Result 1: the problem is not displayed at the bottom of the graph
8. Go to configuration of the trigger and change its expression to: {Test:system.uptime.last()}

   >10s

9. And again, # Open the previously created graph anywhere in frontend.
   Result 2: Now the trigger is displayed at the bottom of the graph.

Expected: Problems that are raised by triggers that use function 'change()' should be displayed on graphs if the 'Show triggers' option is set.

Example: