wxpython as GUI problem

Hello

I write a program which launch pylab from windows GUI (using wxpython).
I found some problem here.

If I use the default backends program in matplotlibrc which is TkAgg
I have this result:
1. I click button the plot is showed
2. I close the plot
3. I click the button to show the plot the second time. it shows the
plot but I can't closed it (it hangs.)

If I use the WX as my backends program ( i have to change manually in
matplotlibrc) I have this result:
1. I click button the plot is showed
2. I close the plot
3. I click the button to show the plot the second time. it shows perfect.
4. I close my GUI this time the program doesn't totally close (I can
check from windows manager process).

What's the problem here. I use pylab.show() to show the program.
I use windows xp sp2

Sincerely Yours,
pujo