Custom Grid header/footer related questions

Flow
1

I have the following in my code:

positionsGrid.addColumn(new ComponentRenderer<>(this::taxAmount))
		.setHeader("Tax")
		.setAutoWidth(true)
		.setFlexGrow(0)
		.setFrozen(true)
		.setSortable(true)
		// here (in the column footer) I would like to display the sum of all amounts from the cells above
		// how do I call a custom method which would return either String or Component (latter is preferred)
		.setFooter(???);