Dialog as transient above current toplevel

Dear all,

in my program I want to provide a more understandable text in the simple dialogs for exiting the
program if there is modified buffer and some other messages.
Instead of creating one dialog for each I use the tkinter dialog Dialog.py like

import Dialog
...
ans = Dialog.Dialog(self.winfo_toplevel(),
	{"title":"Multiple Plot",
	"text":"Continue with the next\nOr do all plots",
	"bitmap": Dialog.DIALOG_ICON,
	"default": 0,
	"strings": ("Next","All","Cancel")})

if ans.num==1:
	doall = True
elif ans.num==2:
	break

However the displayed dialog is centred over the desktop and not above
the running application, which is rather confusing.
Is there a way to center it over my toplevel window?

Thanks in advance
Vasilis