high resolution image - matplotlib

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No idea. But you can still use the figsize argument to obtain a better definition

sage: graphs.PetersenGraph().show(figsize=30)

When you use savefig, set the option dpi to give the resolution in dots per inch.

See the documentation for savefig at:

http://matplotlib.org/api/figure_api.html

You can refer to this online image processing tutorial to see the image resolution solutions, with the mature image resolution adjust methods.

Go for a vector format, like svg or pdf, and then open in, say, Gimp. At the moment of opening, you can specify the resolution easily.