draw legend on the image

def draw(self, img, pixmapper, bounds):
        '''draw legend on the image'''
        if self._img is None:
            self._img = self.draw_legend()

        w = self._img.shape[1]
        h = self._img.shape[0]
        px = 5
        py = 5
        img[py:py+h,px:px+w] = self._img