[ADD] stock: pivot graph for stock forecast

- Task: https://www.odoo.com/web/#id=9161&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.bpogldEeqlHXzCLL
You can remove the string parameter here, as the ORM will generate a capitalized version of the field name by default