Changing Y min/max value from 'item' to other type does not reset the corresponding database fields to null

Steps:
1) Create a graph with Y axis MIN value set to an item.
2) Then change the Y axis MIN value to 'calculated'

Result:
The corresponding graphs.ymin_itemid field still has the old itemid value.

Expected result:
The corresponding graphs.ymin_itemid field must be set to null.