handling pick events doesn't work for me

I have a GTK application with the code

def on_pick(event):
    print "on_pick"
    thisline = event.artist
    xdata, ydata = thisline.get_data()
    ind = event.ind
    print 'on pick line:', zip(xdata[ind], ydata[ind])

cid = fig.canvas.mpl_connect('pick_event', on_pick)

but the routine never gets called when I click the left mouse button.
Instead, the FigureCanvas "pick" method is called.
What am I doing wrong??