Zoom problem when adding a subplot to a figure

Hi list,
I'm working in matplotlib OO mode.

I first create an instance of Subplot(): a = Subplot(f,111) with f an existing figure.
I plot some basic data using a.plot([1,2,3])
Then i add this subplot to the figure with the method f2.add_subplot(a).
The subplot is well displayed in the figure.

But my problem is i can't zoom on the figure. Xlabel and YLabel are redrawn but no the subplot.

Can anybody give me a solution please?
I need to create separately the figure and the subplot.

Thanks a lot,
Philippe Collet