Coordinate translation of tree view cells

[Stefan Weber lawo de]

I'm trying to use 2 GtkTreeViews containing "connectable" items (audio 
inputs/outputs). Between them should be a custom drawing area showing 
connection lines/beziers. So I have to transform the y-component of cell 
coordinates retrieved by gtk_tree_view_get_background_area or 
gtk_tree_view_get_cell_area into the coordinate system of the drawing 
area. Here's some pseudo code (I actually use gtkmm, but anyway):

gtk_tree_view_get_background_area(signal_view,signal_path,signal_column,&signal_rect); 
/* get cell rectangle */

/* now I have tree window coords, but how do I transform them to tree or 
widget coords? */

/* if I had tree coords, I would do the following: */
gtk_tree_view_tree_to_widget_coords(signal_view,signal_rect.x,signal_rect.y,&signal_rect.x,&signal_rect.y);
gtk_widget_translate_coordinates(signal_view,my_drawing_area,signal_rect.x,signal_rect.y,&signal_rect.x,&signal_rect.y);


The point is, it doesn't work. I tried a hack making 
gtk_tree_view_tree_window_to_tree_coords public, because this function 
seemed to solve my problem, but it didn't. The resulting coords are always 
expected_y - tree_view->priv->header_height. By hacking in 
_GtkTreeViewPrivate and adding header_height to the y-component, I get the 
expected result. Did I miss any information, or is it really impossible 
without my hack?

Thanks in advance,
Stefan