Border cells in heatmap

Question

I am trying to construct a heatmap in R using a correlation matrix and a p value matrix.

I use this tutorial to build the heatmap without problems. It works perfectly. I can even introduce some values from a second matrix (the p value matrix) in the right cells.
But when I try to highlight the corresponding cells it doesn't work. I use this code to generate the borders. I use RStudio v0.97 with the packages gplots, RColorBrewer. The code is :

library(gplots)
library(RColorBrewer)

my_palette <- colorRampPalette(c("red", "yellow", "green"))(n = 299)

nx = 5
ny = 10

mat1 <- matrix(rnorm(nx*ny,100,50),nx,ny)
mat2 <- mat1>150 #matrix used for the example instead of the p value matrix

makeRects <- function(tfMat,border){
  cAbove = expand.grid(1:nx,1:ny)[tfMat,]
  xl=cAbove[,1]-0.49
  yb=cAbove[,2]-0.49
  xr=cAbove[,1]+0.49
  yt=cAbove[,2]+0.49
  rect(xl,yb,xr,yt,border=border,lwd=3)
}            #this is the function to make the rectangles/borders
heatmap.2(mat1,
      Rowv = FALSE,         # don't reorganize columns 
      cellnote = mat2,      # check correspondance between rectangles and mat2 values
      main = "Correlation", # heat map title
      notecol="black",      # change font color of cell labels to black
      notecex=0.5,      # change the scaling of the cell labels
      density.info="none",  # turns off density plot inside color legend
      trace="none",         # turns off trace lines inside the heat map
      margins =c(12,9),     # widens margins around plot
      col=my_palette,       # use on color palette defined earlier 
      dendrogram="row",     # don't draw a row dendrogram
      Colv="NA",            # turn off column clustering
      add.expr = {makeRects(mat2,"black")}) #add the borders

I think something is wrong either with the makeRects function, or with the re-ordering of the rows via the heatmap.2 function. The rectangles appear in the heatmap, but they are not at the right positions. I have been scratching my head all day without finding what is wrong.

Any suggestions?