import matplotlib.numerix as npy
from pylab import *

x = npy.arange(1.,10.)
y = arange(1.,10.)
x[4] += 0.4

fig = figure()
ax = fig.gca()

ax.step(x,y, where='post')

y += 1.
ax.step(x,y,where='pre')

y += 1.5
ax.step(x,y,where='mid')

xlim(0,10)
ylim(-1,13)

show()