pos/pos_margin/report/pos_order_report.py

from odoo import fields, models


class PosOrderReport(models.Model):
    _inherit = 'report.pos.order'

    margin_total = fields.Float(string='Margin Total')
    margin_rate = fields.Float(string='Margin Rate', group_operator='avg')

    def _select(self):
        return super(PosOrderReport, self)._select() + """,
    SUM(l.margin) as margin_total,
    (SUM(l.margin / nullif(l.qty,0)) * 100 /
    SUM(nullif(l.purchase_price,0)))::decimal as margin_rate
    """