Plots the genetic map for each chromosome, or a comparison of the
  genetic maps if two maps are given.
For a comparison of two maps, the first
  map is on the left (or, if horizontal=TRUE, on the top).  Lines
  are drawn to connect markers.  Markers that exist in just one map and
  not the other are indicated by short line segments, on one side or the
  other, that are not connected across.
For a sex-specific map, female and male maps are plotted against one
  another.  For two sex-specific maps, the two female maps are plotted
  against one another and the two male maps are plotted against one
  another.