'Handle scroll.'

def on_scroll(self, event):

    scale = (1.1 if (event.step < 0) else (1.0 / 1.1))
    self.cbar.norm.vmin *= scale
    self.cbar.norm.vmax *= scale
    self.cbar.draw_all()
    self.mappable.set_norm(self.cbar.norm)
    self.cbar.patch.figure.canvas.draw()
