def oplot(self,x,y,panel=None,**kw):
        """generic plotting method, overplotting any existing plot """
        if panel is None:
            panel = self.current_panel
        opts = {}
        opts.update(self.default_panelopts)
        opts.update(kws)
        self.panels[panel].oplot(x, y, **opts)
generic plotting method, overplotting any existing plot