Many annotation tracks are actually composed of a number of grouped sub-features, for instance exons in a gene model. This man page highlights the use of grouping information to build informative annotation plots.
All track objects that inherit from class
AnnotationTrack support the grouping feature. The
information is usually passed on to the constructor function (for
AnnotationTrack via the groups argument and for
GeneRegionTrack objects via the exon argument)
or automatically downloaded from an online annotation repository
(BiomartGeneRegionTrack). Group membership is specified
by a factor vector with as many items as there are annotation items in the
track (i.e., the value of length(track). Upon plotting, the grouped
annotation features are displayed together and will not be separated in the
stacking of track items.