I think you need to set an important environment variable. Check this:

C:\>set PEBKAC
Environment variable PEBKAC not defined
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Now if this important var is not defined on your system it should be. Set it like this:

C:\>set PEBKAC=1
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Now all you need to do is:

# for 5.6
ppm install http://theoryx5.uwinnipeg.ca/ppmpackages/GDGraph.ppd

# for 5.8
ppm install http://theoryx5.uwinnipeg.ca/ppms/GDGraph.ppd
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