Python使用matplotlib实现在坐标系中画一个矩形的方法

import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle
class 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()