Steps to reproduce:

1. Apply Cisco IOS by SNMP template (for example) to a host
2. Let zabbix discover Power Supplies (for example) and collect values for 3 days
3. Clic on Latest data
4. Click on Graph near to a power supply item
5. Select now-3d in the time filter

Result:

• The graph is empty and legend shows "no data". 
• Users complain because they can't see graphs for periods > last 2 days.

Expected:

• The graph shows the item values for the 3 days (which actually are in the database, because item history is 7d).

Cause:

• The template has this item prototype configured with "Do not keep trends".
• Zabbix frontend uses trends to generate graphs for periods larger than 2 days.
• Thus these graphs cannot be generated.

Additional note:

Several items in recente official templates have "Do not keep trends", while there seems to be no reason to use this setting.