title: plot multiple shp file on a graph using spplot in R
body: <p>I have 3 shp files representing the house, room, and beds of a house respectively. I need to plot them on a graph using R so that they all overlap with each other. I know that in <code>plot</code> function, I can use <code>line</code> to plot new lines on top of the existing plot, is there anything equivalent in <code>spplot</code>? Thanks.</p>