The bounding box of a layer returns the coordinates of the smallest rectangle that encloses all the elements of the layer.
Examples
if(interactive()){
p <- with(iris, l_plot(Sepal.Length ~ Sepal.Width, color=Species))
l_layer_bbox(p, layer='model')
l <- l_layer_rectangle(p, x=0:1, y=30:31)
l_layer_bbox(p, l)
l_layer_bbox(p, 'root')
}