<pivot string="Tasks Analysis" display_quantity="1" sample="1" disable_linking="1">
                    <field name="project_id" type="row"/>
                    <field name="working_hours_open" widget="timesheet_uom"/>
                    <field name="working_hours_close" widget="timesheet_uom"/>
                    <field name="nbr" invisible="1"/>
                    <field name="rating_avg" invisible="1"/>
                </pivot>