In order to measure a complex impedance from a RF choke or balun, we need to use the S21 mode. Then, under S21, you can select Loss, Phase or delay, so to know the impedance from a choke you need to export the S21 file and use an external application (Excel) to do some maths and get the complex impedance.
You can see what I am talking about in next link:
I wonder if someone can add one more S21 format "Zmag" to perform those measurements without external maths.
Here is the spreadsheet with the calculations:
(OpenOffice file, but opens with Excel as well).
I think it is easy to add and you get an nice new feature.