text: import numpy as npy
  from matplotlib.numerix import npyma as ma
  from matplotlib.pyplot import step, legend, xlim, ylim, show
  
  x = npy.arange(1, 7, 0.4)
  y0 = npy.sin(x)
  y = y0.copy() + 2.5
  
  step(x, y, label='pre (default)')
  
  y -= 0.5
  step(x, y, where='mid', label='mid')
  
  y -= 0.5
  step(x, y, where='post', label='post')
  
  y = ma.masked_where((y0>-0.15)&(y0<0.15), y - 0.5)
  step(x,y, label='masked (pre)')
  
  legend()
  
  xlim(0, 7)
  ylim(-0.5, 4)
  
  show()
  
  