It would be great to be able to create or use a custom source on report.
The problem arises every time we want to visualize data from different processes on a single chart. In such cases, the default sources are limited to system data only. It is not possible to physically connect logically related data.
Our case is a little more complex.
We have long-term tasks, connected hierarchically, whose progress is recorded in lists of items within the tasks themselves. We want to show a chart of task progress at the highest level, taking into account subordinate tasks. So we need to refer to different processes, but also retrieve data from their item lists.
It is possible to achieve this functionality using a chart or data table attribute in process, but there is no equivalent functionality at the report level.
I think that the ability to use our own source, even if it exists in Designer, would significantly expand the possibilities for data visualization.