[R] locator() in a multi-figure setting using mfrow(): SOLVED

[Christoph Lehmann]

thanks to some great hints by Paul Murrel I could solve it: here we are 
with one solution (code needs to be cleaned and simplified, but maybe 
one can understand it)

#######
## create a multifigure setting
nr <- 4
nc <- 2
opar <- par(mfrow = c(nr, nc))
slices <- 8
m <- matrix(runif(100),10,10)
my.list <- list()
for (slice in 1:slices) {
     my.list[[slice]] <- m
}

for  (slice in 1:slices) {
     x <- 1*(1:25)
     y <- 1*(1:25)
     z <- my.list[[slice]]
     image(list(x = 0:9, y = 0:9, z = z))
}
########
my.get.coord <- function() {
par(mfg = c(1,1)) #locator() shall be relative to the first plot out
# of the eight plots totally
my.loc <-locator(1) #location, not in inches
my.plot.region <- par("usr") #extremes of plotting region
#(in plot units, not inches)
my.plot.region.x <- my.plot.region[2] - my.plot.region[1]
my.plot.region.y <- my.plot.region[4] - my.plot.region[3]
my.loc.inch.x <- (my.loc$x + 0.5)/my.plot.region.x * (par("pin")[1]) 
#par("pin") #current plot dimension in inches
#relative to the plotting-region bottom left corner, not the axis c(0,0) 
point
my.loc.inch.y <- (my.loc$y + 0.5)/my.plot.region.y * (par("pin")[2])

## search the plot we are in with locator(1)
my.plot.inch.x <- par("pin")[1] + par("mai")[2] + par("mai")[4] #plot.x 
+ left & right margin
par("fin")[1]
my.plot.inch.y <- par("pin")[2] + par("mai")[1] + par("mai")[3] #plot.y 
+ bottom & top margin
par("fin")[2]

pos.rel.x <- (my.loc.inch.x / par("fin")[1] - floor(my.loc.inch.x / 
par("fin")[1])) *
      par("fin")[1] / par("pin")[1] * (par("usr")[2] - par("usr")[1]) - 0.5
      #inches from left bottom corner in target plot region (c(0,0)
      # is plot-region bottom-left corner, not the axis c(0,0) point
pos.rel.y <- (my.loc.inch.y / par("fin")[2] - floor(my.loc.inch.y / 
par("fin")[2])) *
      par("fin")[2] / par("pin")[2] * (par("usr")[4] - par("usr")[3]) - 0.5
      #inches from left bottom corner in target plot

fig.coord.x <- ceiling(my.loc.inch.x / par("fin")[1])
fig.coord.y <- 1 +(-1) *ceiling(my.loc.inch.y / par("fin")[2])
# cat("figure-coord x: ", fig.coord.x,"\n")
# cat("figure-coord y: ", fig.coord.y,"\n")
cat("we are in figure: ", fig.coord.y * nc + fig.coord.x, "\n")
cat("coordinates of the identified point x: ", round(pos.rel.x),"\n")
cat("coordinates of the identified point y: ", round(pos.rel.y),"\n")
}

########
my.get.coord()


Christoph