[R] Using latticeExtra as.layer function with different number of plot panels

[sbihorel]

Hi,

When the as.layer function is used to overaly 2 lattice plots, there 
seems to be an assumption that the data used in both plots will generate 
the same number of panels (and, I believe, in the same order). In case 
the data used in the plot within the as.layer call is incomplete , data 
may be plotted on the "wrong" panel, and data seem to get re-used on the 
last panel(s). See what happens in the example code below when the 
records with state.region=="South" are dropped...

Is there a trick to overlay panel based upon the conditioning variable 
value rather than the panel order?

require(lattice)
require(latticeExtra)
state2 <- state <- data.frame(state.x77,state.region)
state2$Income <- sample(state2$Income)
state3 <- state2[which(state2$state.region!="South"),]
foo <- xyplot(Income~Population|state.region,data=state,main='foo')
foo

bar <- update(foo,main='bar') + 
as.layer(xyplot(Income~Population|state.region,data=state2,col='red'))
bar

bar2 <- update(foo,main='bar2') + 
as.layer(xyplot(Income~Population|state.region,data=state3,col='red'))
bar2

Thank you

Sebastien

PS: I know that I could get what I want by setting the Income variable 
to NA for records with state.region=="South" instead of dropping them... 
but this is not the point of my example. I am just trying to illustrate 
what happens when as.layer is used for plotting data with inconsistent 
dimensions.