Shared axis question

Consider the following code (adapted from

from pylab import *
t = arange(0.01, 5.0, 0.01)
s1 = sin(2*pi*t)
s2 = sin(4*pi*t)
s3 = sin(6*pi*t)

ax1 = subplot(311)
ax2 = subplot(312, sharex=ax1)
plot(t, s2)
ax3 = subplot(313, sharex=ax1)
plot(t, s3)

When I use the zoom feature of the NavigationToolbar, the xaxis is shared between all 3 graphs.
But the yaxis limits are free. Obviously, they could also be shared but this is not want I want.
I would like to "force" or "freeze" the Y axis to a given value. Consequently, the zoom feature would act only
along the X axis.

Any ideas??
Clovis Goldemberg
University of Sao Paulo