All items that have a return value of "unixtime" have broken graphs. They sho a horizontal line instead of a continiously rising line.
https://www.zabbix.com/forum/showthread.php?t=44285

This can easily be teste with an item with key "system.localtime" and "unixtime" as unit. The produced graph is always a horizontal line no matter how often the item returns values.
The values are correct (when selected with "latest data" and "show values". Just the graphing os broken,