Steps to reproduce:

1. Create a host
2. Create three items: "a", "b", "c"
3. Create a normal graph type (or stacked) by adding item "a" as "Y axis MIN value", "b" as "Y axis MAX value" and "c" to item list.
4. Save graph.
5. Open graph to edit it.
6. Press clone, change name and graph type to "Pie".
7. Leave that "c" item where it is.
8. Observe "a" and "b" Y MIN/MAX items are gone.
9. Save graph.
10. Export host.
11. Import host back.

Result:
"Invalid tag "/zabbix_export/graphs/graph(2)/ymin_item_1": unexpected tag "host"."

Expected:
Y MIN/MAX items should be cleared when changing graph type during cloning, so they are not exported at all and so that import works successfully.