If you look at the attached graph, you can see that the y-axis shows 1.86GB for two separate lines. Another thing that shows an issue is that the "max" value in the graph for free space says that the max is 6.36GB, but the graph doesn't relect that. If I move the graph forward an hour, the duplicate y-axis entry problem does not occur and the graph shows the correct values.
Another thing to point out is that the highest value on the y-axis is garbled. It looks like Zabbix tried to write two things there.