portfolio.py: savefig() after show() will cause a empty graph

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What steps will reproduce the problem?
1. portfolio.plot()
2. portfolio.show()
3. portfolio.savefig("test.png")

What is the expected output? What do you see instead?
test.png should be the portfolio graph. But I get an empty png instead

Please use labels and text to provide additional information.
This is a known issue of matplotlib:
http://stackoverflow.com/questions/5158447/matplotlib-pyplot-show-doesnt-work-on
ce-closed

so make sure to call savefig() first before call show(). Or wait util the next 
version > 1.0.1 of matplotlib

Original issue reported on code.google.com by panpandas on 8 May 2011 at 1:00