You have pylab.ylim
:
pylab.ylim([0,1000])
Note: The command has to be executed after the plot!
Update 2021
Since the use of pylab is now strongly discouraged by matplotlib, you should instead use pyplot:
from matplotlib import pyplot as plt
plt.ylim(0, 100)
#corresponding function for the x-axis
plt.xlim(1, 1000)