Using mouse to plot points on image in Python, with Matplotlib

I am trying to use the mouse to plot points on an image. The problem: the image appears, but when I click on the mouse, nothing is plotted (even if I click several times).

My version of Python is Python 2.7, with Anaconda and the IPython console. I type %pylab in the Ipython console before running the script.

Here is my code:

import numpy as np
from matplotlib import pyplot as plt 
#Some code here [. . .]

fig, ax = plt.subplots(1)
ax.imshow(img, interpolation = 'bicubic')
'''preventing plot from rescaling image:'''
ax.set_xlim([0.0, img.shape[1]])
ax.set_ylim([img.shape[0], 0.0])
ax.hold(True)
ax.autoscale = False
#ax.plot(100,100, 'ro')  # This works

class MouseMonitor:
    flag = True
    x = 0.
    y = 0.  
    fig = None
    axes = None

    def __init__(self, fig, ax):
        self.axes = ax
        self.fig = fig
    def __call__(self, event):
        if self.flag:
            print('({}, {})'.format(event.xdata, event.ydata))
            self.flag = False
        else:
            d = np.linalg.norm([event.xdata - self.x, event.ydata - self.y])
            print('({}, {})\n\n distance between points: {} m\n\n-------------------\n'.format(event.xdata, event.ydata, d))
            self.flag = True                       
        self.x = event.xdata
        self.y = event.ydata
        self.axes.plot(self.y, self.x, 'ro', linewidth = 5) #This don't work

mouse = MouseMonitor(fig, ax)

cid = fig.canvas.mpl_connect('button_press_event', mouse)