Python使用matplotlib实现在坐标系中画一个矩形的方法

import matplotlib.pyplot as pltfrom matplotlib.patches import Rectangleclass Annotate(object):  def __init__(self):    self.ax = plt.gca()    self.rect = Rectangle((0,0), 1, 1)    self.x0 = None    self.y0 = None    self.x1 = None    self.y1 = None    self.ax.add_patch(self.rect)    self.ax.figure.canvas.mpl_connect('button_press_event', self.on_press)    self.ax.figure.canvas.mpl_connect('button_release_event', self.on_release)  def on_press(self, event):    print 'press'    self.x0 = event.xdata    self.y0 = event.ydata  def on_release(self, event):    print 'release'    self.x1 = event.xdata    self.y1 = event.ydata    self.rect.set_width(self.x1 - self.x0)    self.rect.set_height(self.y1 - self.y0)    self.rect.set_xy((self.x0, self.y0))    self.ax.figure.canvas.draw()a = Annotate()plt.show()