Problem:

Graph prototype is unable to generate a graph from the discovery result with only one element in the array if the graph prototype's name doesn't include a macro.

Steps to reproduce:

Create (for example) a SNMP discovery rule which output is an array with only one element:

Rule's SNMP OID:

discovery[{#FOO},1.3.6.1.x.x.x.x]

Output: 

[{"{#SNMPINDEX}":"x.x.x.x","{#FOO}":"BAR"}]

Create item prototype.

Create graph prototype named "Value".

Result:

Error message: "Cannot update graph: graph with the same name "Value" already exists."

Workaround:

If the graph prototype's name has a macro in it ("{#FOO} Value"), the one and only graph will be generated.