r - taking difference of two xyplots? - r

Question

I have several xyplot objects that I have saved as .RDATA files. I am now interested in being able to look at their differences. I have tried things like
plot1-plot2
but this does not work (I get the "non-numeric argument to binary operator error).
I would also be able to do this if I knew how to extract the timeseries data stored within the lattice xyplot object, but I have looked everywhere and can't figure out how to do this either.
Any suggestions?
EDIT:
just to make it perfectly clear what I mean for MrFlick, by "taking the difference of two plots" I mean plotting the elementwise difference of the timeseries from each plot, assuming it exists (i.e. assuming that the plots have the same domain). Graphically,
I might want to take the following two plots, stored as xyplot objects:
and end up with something that looks like this:
-Paul

Answer 1

Here is a little function I wrote to plot the difference of two xyplots:
getDifferencePlot = function(plot1,plot2){
data1 = plot1$panel.args
data2 = plot2$panel.args
len1 = length(data1)
len2 = length(data2)
if (len1!=len2)
stop("plots do not have the same number of panels -- cannot take difference")
if (len1>1){
plotData = data.table(matrix(0,0,4))
setNames(plotData,c("x","y1","y2","segment"))
for (i in 1:len1){
thing1 = data.table(cbind(data1[[i]]$x,data1[[i]]$y))
thing2 = data.table(cbind(data2[[i]]$x,data2[[i]]$y))
finalThing = merge(thing1, thing2,by = "V1")
segment = rep(i,nrow(finalThing))
finalThing = cbind(finalThing,segment)
setNames(finalThing,c("x","y1","y2","segment"))
plotData = rbind(plotData,finalThing)
}
}
if (len1==1){
plotData = data.table(matrix(0,0,3))
setNames(plotData,c("x","y1","y2"))
thing1 = data.table(cbind(data1[[i]]$x,data1[[i]]$y))
thing2 = data.table(cbind(data2[[i]]$x,data2[[i]]$y))
plotData = merge(thing1, thing2,by = "V1")
}
plotData$difference = plotData$y1-plotData$y2
if (len1==1)
diffPlot = xyplot(difference~x,plotData,type = "l",auto.key = T)
if (len1>1)
diffPlot = xyplot(difference~x|segment,plotData,type = "l",auto.key = T)
return(diffPlot)
}