'This is where you actually build the GUI widgets (called by
__init__).  The icons ``home.xpm``, ``back.xpm``, ``forward.xpm``,
``hand.xpm``, ``zoom_to_rect.xpm`` and ``filesave.xpm`` are standard
across backends (there are ppm versions in CVS also).
You just need to set the callbacks
home         : self.home
back         : self.back
forward      : self.forward
hand         : self.pan
zoom_to_rect : self.zoom
filesave     : self.save_figure
You only need to define the last one - the others are in the base
class implementation.'

def _init_toolbar(self):

    raise NotImplementedError
