def onMargins(self, left=0.1, top=0.1, right=0.1, bottom=0.1):
        """ pass left/right margins on to bottom panel"""
        bconf = self.panel_bot.conf
        l, t, r, b = bconf.margins
        bconf.set_margins(left=left, top=t, right=right, bottom=b)
        bconf.canvas.draw()