Grob
gets grob object supported by grid system, and make an instance of
subclass of class 'Grob'.'GrobList' is a container of list of 'Grob' object.
## S3 method for class 'gg':
Grob(x)
## S3 method for class 'gtable':
Grob(x)
## S3 method for class 'trellis':
Grob(x)
## S3 method for class 'lattice':
Grob(x)
## S3 method for class 'GGbio':
Grob(x)
Grob
object.