Drawing Contour Plots (http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl?Drawing_Contour_Plots). A lesson plan that deals with drawing various isopleths including isotachs.
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... Ross J Anderson on his Web site at www.cl.cam.ac.uk/~rja14/tcpa-faq.html/ for a clear discussion of two prominent proposals.
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