mfg, mfcol, mfrow, and layout() - secret friends

I was working on an issue (enhancement) today in my groan R-package today that required adding additional plotting elements via lines() and points() to a device that had already been partitioned by layout(). The code I wanted to use was essentially:

# Y and S are lists of xy.coords() objects of the same length

lyt = matrix(1:length(Y), ncol=10)
layout(mat=lyt)

# function A
# plot Y first as points
lapply(Y, function(x) {
  plot(x, ...)
})

# function B
# overlay S as lines on the grid of plots for Y
lapply(S, function(x){
  lines(x, ...)
})

However, I would only get all of the above lines in one subplot. For a brief moment, I considered rewriting my whole set of plotting methods to use split.screen() or par(mfcol). Ugh!

On a whim, I decided to check what par('mfg') would return after a device had been partitioned and plotted to with:

layout(matrix(1:9, nrow=3))
par(mar=c(0,0,0,0))
plot(runif(10))

I was pleasantly surprised to find:

> par('mfg')
[1] 1 1 3 3

indicating that I could potentially direct the next plot in a layout()'ed device by setting the value of mfg= to the next plot id:

lyt = matrix(1:9, nrow=3)
par(mfg=which(lyt == NextPlotID, arr.ind=TRUE)[1,])

Unicorns and rainbows, this works! (despite all the dire warnings in the documentation regarding incompatibilities)

Thus, the resulting code:

# Y and S are lists of xy.coords() objects of the same length

lyt = matrix(1:length(Y), ncol=10)
layout(mat=lyt)

# function A
# plot Y first as points
lapply(Y, function(x) {
  plot(x, ...)
})

# function B
# overlay S as lines on the grid of plots for Y
lapply(seq_along(S), function(n){
  par(mfg=which(lyt == n, arr.ind=TRUE)[1,]) # sets next plot in grid!
  lines(S[[n]], ...)
})

and issue resolved.

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