cellGrob: Get a grob describing the content of an assemble_cell object
Description
Methods for this generic should be defined for all assemble_cell subclasses
and should return a compliant gtable object ready to be combined with
regular plot objects. In general it is best to call cell_table() on the
object and add grobs to this as cell_table() will return a compliant
gtable
Usage
cellGrob(x, guides = "auto")