These functions help detect the placement of panels in a gtable, if they are
named with "panel" in the beginning. find_panel returns the extend of
the panel area, while panel_cols and panel_rows returns the
columns and rows that contains panels respectively.
find_panel(table)panel_cols(table)
panel_rows(table)
A gtable
A data.frame with some or all of the columns t(op), r(ight), b(ottom), and l(eft)