Steps to reproduce:

1. Create a discovery rule.
2. Create 2 or more item prototypes in it (e.g. Item #1, Item #2 and Item #3).
3. Create a graph for all item prototypes.
4. Create an override for one item prototype to set Discover = No.
   
5. Execute now the discovery rule.
6. Items from prototypes have been created (without Item #2 in my example). So far so good.

Result:

No graph is created (at least with Item #1** and Item #3 in my example).

Expected:

Graph prototype should also be discovered even if an item prototype is not or no longer discovered. Especially when you work excessively with singleton discoveries in order to filter out unsupported items, this is extremely annoying. If I really don't need a graph, I can still filter it out via a separate override. It would really be better if graphs are created as long as at least one item prototype is still available.

Workaround:

You can work around the error by having the discovery rule discover all items first. Then let the override not discover the corresponding item anymore. Then remove the orphaned item manually. Now the graph itself survives, especially after a re-run of the discovery rule. However, as soon as there is a change in the graph (item is added, changed or removed), the graph is unfortunately no longer discovered and is then deleted anyway. So that's not really a solution either.