Steps to reproduce:

1. Navigate to Configuration -> Hosts -> Graphs
2. Create a graph and specify some item as Y min value.
3. Save changes
4. Open same graph for editing
5. Delete selected host and item from "Y axis MIN value" field (or type in some invalid value)
6. Click "Update"

Result:
Message tells that graph is updated. Opening a graph form again, see that changes are not made.
Expected:
Validation should check if mandatory field is filled.