'return Nx2 array of x,y coordinate of the boundary'

def get_boundary(self):

    (x0, x1, y0, y1) = self._extremes
    tr = self._aux_trans
    xx = np.linspace(x0, x1, 100)
    (yy0, yy1) = (np.empty_like(xx), np.empty_like(xx))
    yy0.fill(y0)
    yy1.fill(y1)
    yy = np.linspace(y0, y1, 100)
    (xx0, xx1) = (np.empty_like(yy), np.empty_like(yy))
    xx0.fill(x0)
    xx1.fill(x1)
    xxx = np.concatenate([xx[:(-1)], xx1[:(-1)], xx[(-1):0:(-1)], xx0])
    yyy = np.concatenate([yy0[:(-1)], yy[:(-1)], yy1[:(-1)], yy[::(-1)]])
    t = tr.transform(np.array([xxx, yyy]).transpose())
    return t
