Transparency disappears when saving a figure in Postscript

Hi, 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)

Postscript does not support transparency -- this is a limitation of
postscript and not the matplotlib ps backend. If you want vector
graphics with transparency, you'll need SVG or PDF.