[R] polygons around clusters of identically valued nodes in levelplot()

[Scott Waichler]

I'm looking for a way to plot lines on top of a levelplot(),
where the lines are borders between cells of different values.
The clines() function provides contours suitable for continuous
data.  I am dealing with discrete values, spatial clusters of nodes 
where each cluster has an integer value, and I want to plot the 
borderlines between these areas.
So, in a levelplot having nodes with values 0, 1, or 2, I would
like to plot lines between nodes having the values 0 and 1, 0 and 2 and 0 and 2.
The contouring concept gives two lines for adjacent nodes having values
of 0 and 1.

Thanks,
Scott Waichler
Pacific Northwest National Laboratory
Richland, Washington, USA
scott.waichler at pnl.gov