two itens:

item of the zabbix agent type shows correct on graph (Image: correct-type-zabbix-agent.png)

item of the zabbix traper type shows on graph filled with all the timestamps (Image: bugged-type-zabbix-trapper.png)

I also atached the database info of the itens. They have the same data. Anyway I change the type of the zabbix traper item to zabbix agent and it make show correct on graph (same item).
So I think is something with the graph code.