[R] Using a constant scale across X-Y plots

[Lisa]

I am working off an example from Deepayan Sarkar's  
Lattice:Multivariate Data Visualiization with R. I am trying to create  
Figure 5.6, essentially, but I would like to be able depict different  
metro areas.  These of course  have different lat/longs, so I need to  
make different graphs.  BUT (and this has been the tricky part), I  
would like to produce graphics for each metropolitan region using the  
same depth scale for all, so that dark gray means the same thing  
consistently.

  That's the part that is proving way too hard for my little brain and  
limited R experience.

I'm assuming I don't have to include the quakes data to play around  
with.

depth.col <- gray.colors(100)[cut(quakes$depth, 100, label = FALSE)]

depth.ord <- rev(order(quakes$depth))

depth.breaks <- do.breaks(range(quakes.ordered$depth), 50)

quakes.ordered$color <- level.colors(quakes.ordered$depth, at =  
depth.breaks, col.regions = gray.colors)

xyplot(lat ~ long | Magnitude, data = quakes.ordered, aspect = "iso",  
groups = color, cex = 2, col = "black", panel = function(x, y,  
groups, ..., subscripts) { fill <- groups[subscripts] panel.grid(h =  
-1, v = -1) panel.xyplot(x, y, pch = 21, fill = fill, ...) }, legend =  
list(right =
list(fun = draw.colorkey, args = list(key = list(col = gray.colors, at  
= depth.breaks), draw = FALSE))), xlab = "Longitude", ylab = "Latitude")

Any ideas how how I might approach this?

Many thanks.
Lisa