Transparency disappears when saving a figure in Postscript


below is a simple script illustration what I wish to do:
    ==> use new symbols (here ellipses for example) and plot a number of
points with 'scatter', allowing transparency (alpha=0.2) to see when two
data points overlap.
For some reason this works: 1/ on the figure 2/ when saving as .png
but it does NOT work when saving as a Postscript file.

Is there an easy solution for this? (I am using GTKAgg, mpl 0.87.5)

(doing a "convert" afterwards works but this is not very practical here)

thanks in advance

P.S.: for the example above, just using standard circles would have the
same effect


import numpy as num

rx, ry = 1.8, 1.
area = rx * ry * num.pi
theta = num.arange(0, 2*num.pi+0.01, 0.1)
verts = zip(rx/area*num.cos(theta), ry/area*num.sin(theta))

x = [0,0.1,0.2, 0.5]
y = [0.,0.1,0.,0.2]
scatter(x,y, c='0.8', edgecolor='k', faceted=True, s=300, marker=None,
verts=verts, alpha=0.2)
## saving as a png
## the figure shows the transparency well

## now saving as a postscript
## ==> the symbols overlap but the transparency is gone


Eric Emsellem emsellem@...419...
                           Centre de Recherche Astrophysique de Lyon
9 av. Charles-Andre tel: +33 (0)4 78 86 83 84
69561 Saint-Genis Laval Cedex fax: +33 (0)4 78 86 83 86