gnome canvas and text item

[Marco Mussini <marcomus libero it>]

If I insert a text item in a gnome canvas with function:

gnome_canvas_item_new (group,
                               gnome_canvas_text_get_type (),
                               "text", "My text",
                               "x", 10.0,
                               "y", 10.0,
                               "font", "Sans 30",
                               "justification", GTK_JUSTIFY_CENTER,
                               "fill_color_rgba", "0x0000ff80",
                               NULL);

and after try to change zoom factor with:
gnome_canvas_set_pixels_per_unit(GNOME_CANVAS (canvas), 3.0);
the text height don't change. 
Is there a metod that update automatically text height in function of zoom 
factor ?
Sorry for my english!
Thank you,
Marco