Steps to reproduce:

1. On a dashboard, create a widget of type Graph (NOT classic).
2. Add a data set with a host pattern and an item pattern, that matches multiple hosts and in my case on item on each of those hosts.
3. Choose "Aggregate function = sum" and "Aggregate = Data set"

Result:
The graph doesn't show the sum, but (I guess) only the value of the first item.

Screenshot #1 with Aggragate function sum of the data set:

Screenshot #2 without Aggregate function, but stacked values of each item: