There is no scale as such. The mathematics provides the translation from the rectangular Z coordinate system to polar and the result of that math transformation yields the curves your are referencing. We and the vna actually only deal with one of the four Chart regions. The Chart actually occupies 4 quadrants.
In 2001, I wrote a paper that constructed the chart and built it in Mathcad. The paper appeared in Applied Microwave. Each piece of the chart, the real axis and the imaginary contours were assembled as individual blocks of these mapping routines. See reference that should provide a link to the paper.
Alan
