Display subtotal differences in the OO ALV grid

I am implementing an ALV grid of purchase order history. The requirement is to display the quantity differences of subtotals between goods receipts and invoice receipts at the end of the ALV grid. Ideally, this needs to be displayed in a row similar to the total row.

I can display and obtain the subtotals using the subtotal_text event and <grid>-get_subtotals method. Hence, I can calculate the differences of subtotals.

How can I display the difference row in the ALV?

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GR1

GR2

GR3

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GR Subtotals:

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IR1

IR2

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IR Subototals:

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Quantity differences: <-- how to display this row at the end of the grid?

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Thanks in advance.

Ning