disabling autoscaling

Hi,

let me try another one:

···

#-----------------------
from pylab import *

fig = figure(figsize=(6,6))
ax = fig.add_axes([0.1, 0.1, 0.8, 0.8])

phi=arange(0.0,2.0*pi+0.05,0.1)
x=cos(phi)
y=sin(phi)
plot(x,y,linewidth=2)
xlim(-1.1,1.1 )
ylim(-1.1,1.1 )

# Question: how can I turn-off autoscaling,
# so that the circle stays a circle
# and the following plot only adds a part
# of an ellipse?
plot(2*x,0.5*y,linewidth=5)

show()
#------------------------

So the question is, how can I turn off
auto-scaling and just leave the plot window
to the specified rectangle [-1.1,1.1]^2 ?

Many thanks,

Arnd