Semi transparent boxes around text and Postscript

The that the polar plot overlayed on a rectangular plot is working in Postscipt, I am having trouble with the semi-transparent boxes around the text.

The png file looks fine, but the postcript file does not show the differences in the alpha value.

Since the code is short, I'll supply it here:

#!/usr/bin python
from matplotlib.matlab import *

def OverlayText(xmax):
  y = 0.65*xmax
  ystep = 0.2*xmax
  alpha = arange(0.0,1.2,0.2)

  for a in alpha:
    stext = 'Alpha = %.1f' % a
    t = text(x,y,stext)
    y = y - ystep

if __name__ == "__main__":
  # Make some kind of plot

  xmin = -750
  xmax = 750
  ymin = xmin
  ymax = xmax
  x = [xmin,xmax,xmax,xmin]
  y = [ymin,ymin,ymax,ymax]
  a = gca()





Ms. Carol A. Leger
SRI International Phone: (650) 859-4114
333 Ravenswood Avenue G-273
Menlo Park, CA 94025 e-mail: leger@...392...