I attempted this a few months ago and I don't think it's possible even with editing the chart xml. It's pretty easy to do using a pivot chart in Excel and you could save the Excel doc as a report in CRM. Alternatively if you need this presenting within
a CRM form or dashboard, then you'll need to write an SSRS report.
If you are intent on using crm charts, then you'll need to construct a data model that can store the cumulative values but it's a bit messy as you'd need to build something to calculate and update these records at the end of each month. Using CRMOnline,
your options in this respect are a bit limited at the moment.
Microsoft Certified Technology Specialist (CRM) GAP Consulting Ltd Microsoft Community Contributor Award 2011