to come back on this, I think we our self found a solution to our problem.
we created a custom template for the report, based on an existing one.
In the reportgraph section, we changed the parameters of the graph, and added startoff="-90" endoff="0"
apparently these parameters control the start/enddate offset from what the chosen date range is in the setup.
so now, every first day of the month, we generate a report period type month, with report period previous.
If we run this for the period 1/9-30/9....with the offset calculated, it will give us a graph from 1/6..30/9....just what we wanted.
We had to figure out some stuff in the report templates, but the result is there so now we can start using this...
Thnx for the previous help, but as we resolved the problem, this thread can be closed.