I recommend removing the "Close the Figure" button from the toolbar.
It just destroys the widget / exits the program, which you can already
do by clicking the close button that the window manager provides.
The GNOME Human Interface Guidelines, Chapter 5 Toolbars, says
"Don't add buttons for Help, Close or Quit to your toolbar by default,
as these are rarely used and the space is better used for more useful
controls."
http://developer.gnome.org/projects/gup/hig/1.0/toolbars.html
This reference is for GNOME/GTK+ applications and other windowing
environments will have their own guidelines, but I think they agree that
Close/Quit buttons on toolbars are not needed.
Check your own system - run some applications you use that have toolbars
and check if there is a quit button, and even if there is would it cause
a problem if it wasn't there?
Steve