pos/pos_pricelist/views/point_of_sale_view.xml

<?xml version="1.0"?>
<openerp>
    <data>

        <record id="view_pos_pos_form" model="ir.ui.view">
            <field name="name">pos.order</field>
            <field name="model">pos.order</field>
            <field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>
            <field name="arch" type="xml">
            	<xpath expr="//field[@name='lines']/tree/field[@name='discount']" position="after">
            		<field name="tax_ids" widget="many2many_tags"/>
            	</xpath>
			</field>
		</record>

    </data>
</openerp>