python - Prints value when clicked on but line isn't created? -

i'm learning matplotlib library on python 3.4. i'm practicing on code user supposed interact plot clicking on it. when click happens somewhere on plot 2 things supposed happen: print out y value , create horizontal line in plot click happened.

what i'm getting when click somewhere on plot print happens desired horizontal line shows if press f (which fulls screen plot). in other words, line created when click on plot doesn't appear untill press f. ideas why it's happening?

the code:

import matplotlib.pyplot plt import numpy np  fig = plt.figure() ax = fig.add_subplot(111) ax.plot(np.random.rand(10))  def pick(event):     plt.hlines(event.ydata,event.xdata-     0.2,event.xdata+0.2,colors='r',linestyle='solid')     print('y coord = %f'%event.ydata)  fig.canvas.mpl_connect('button_press_event',pick) plt.show()     

you need use interactive mode update plot after each click. use plt.ion() before plt.show().