I posted this question to gnome-devel-list:

> I don't understand the ordering commands.
> gnome_canvas_item_raise_to_top() and
> gnome_canvas_item_lower_to_bottom() are clear and useful, but
> gnome_canvas_item_{lower,raise}() are not very useful because they
> work in "positions".  I don't know the relative positions of objects
> (there doesn't seem to be an API to extract your stacking order).  An
> API where you specify positions relative to some other object would be
> much more useful, IMHO.  For example, Tk's says "lower item X below
> other item Y".
> 
> Should we either export an item's stacking position (to make
> gnome_canvas_item_lower useful if painful), or add APIs that allow you
> to do relative position?  (Perhaps
> gtk_canvas_item_lower_relative(GnomeCanvasItem *item_to_lower,
> GnomeCanvasItem *superior_item), where after it, item_to_lower is one
> stacking step lower than superior_item)?


Getting no reply, I went ahead and implemented the function I wanted.
I don't see any way to (easily) provide this functionality without access
to libgnomecanvas internals,
so I'd love to see this code in a future release.

   -John Heidemann

/**
 * gnome_canvas_item_lower_below:
 * @item: A canvas item whose stacking order should change.
 * @superior: The item that will be just above ITEM upon exit.
 *
 * Raises the item in the stacking order so that it just just below
 * superior.
 **/
void
gnome_canvas_item_lower_below(GnomeCanvasItem *item, GnomeCanvasItem *superior)
{
	GList *item_link, *superior_link;
	GnomeCanvasGroup *parent;

	g_return_if_fail(GNOME_IS_CANVAS_ITEM(item));
	g_return_if_fail(GNOME_IS_CANVAS_ITEM(superior));

	if (!item->parent)
		return;  /* not part of a canvas? */
	if (!superior->parent)
		return;  /* not part of a canvas? */
	g_assert(item->parent != superior->parent);  /* can't move between canvases */

	parent = GNOME_CANVAS_GROUP(item->parent);
	item_link = g_list_find(parent->item_list, item);
	g_assert(item_link != NULL);
	superior_link = g_list_find(parent->item_list, item);
	g_assert(superior_link != NULL);

	/* move it */
	if (put_item_after(item_link, superior_link)) {
		redraw_if_visible(item);
		item->canvas->need_repick = TRUE;
	};

}