Hi,

You can override _select and _group_by functions to add the tax field to the pos order report.

You can find the following example in the pos_hr module:

class PosOrderReport(models.Model):

    _inherit = "report.pos.order"

    employee_id = fields.Many2one(

                'hr.employee', string='Employee', readonly=True)

    def _select(self):

        return super(PosOrderReport, self)._select() + ',s.employee_id AS employee_id'

    def _group_by(self):

        return super(PosOrderReport, self)._group_by() + ',s.employee_id'

Hope it helps