How add *semilogy* plot to a Figure object?

It appears when I use add_subplot method to add a plot

    > to a Figure object that it is NOT semi-logarithmic if I
    > use semilogy method later.

Works for me...

import matplotlib.numerix as nx
from pylab import figure, show
fig = figure()
ax = fig.add_subplot(111)
x = nx.arange(0.01, 5.0, 0.01)
y = nx.exp(-x)