Add reference boxes around each cell of the glyphmap.
add_ref_boxes(data, var_fill = NULL, color = "white", size = 0.5,
fill = NA, ...)
A glyphmap structure.
Variable name to use to set the fill color
Set the color to draw in, default is "white"
Set the line size, default is 0.5
fill value used if var_fill
is NULL
other arguments passed onto geom_rect