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)

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.

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.