Adding Tax to Lines in a CXML Invoice

I am trying to process a multi-line invoice using a CXML file type where 1 of the lines is taxable and the other line is tax-free.  What should be the file layout to include VAT/GST at the line level on the invoice.  If I add the tax to the summary level, it assumes the entire invoice is taxable, and I receive an error stating the calculated tax does not agree with the entered tax amount