def add_arrow(self, x1, y1, x2, y2,  side='left',
                  shape='full', color='black',
                  width=0.01, head_width=0.03, overhang=0, **kws):
        """add arrow supplied x, y position"""
        dx, dy = x2-x1, y2-y1

        axes = self.axes
        if side == 'right':
            axes = self.get_right_axes()
        axes.arrow(x1, y1, dx, dy, shape=shape,
                   length_includes_head=True,
                   fc=color, edgecolor=color,
                   width=width, head_width=head_width,
                   overhang=overhang, **kws)
        self.draw()