How to plot circular chromosomes with Gviz?
The primary author of Gviz confirmed to me via email that this is not possible:


  Gviz will only give you the traditional linear tracks. There are a
  bunch of solutions available for visualizing circular genomes, most of
  which are more interactive in nature and not implemented in the rather
  static R graphics context. GGplot2 has some support for circular axes,
  too. If you want to stay within the R environment I’d start there.
  Ggbio is another option providing a somewhat higher-level interface.