Account invoice report

<openerp>
<data>

    <template id="report_invoice_document" inherit_id="account.report_invoice_document">

        <xpath expr="//span[@t-field='t.amount']" position="after">
            <span t-field="t.note"/>
        </xpath>
        </template>
    </data>
</openerp>

i added field to invoice report inside tax table. but how can i make tax table visible only if there is note field, and hide if note is empty.

i trying something with t-if but my goal is to show tax table not to hide it when note field is not empy. is there any kind of t-ifnot?

<xpath expr="//span[@t-field='t.amount']/../../../../thead/tr" position="replace">
                <th t-if="o.notes"

            </xpath>