[R] strange tickmarks placing in image

[Arne.Muller@aventis.com]

Hello,

I've a problem aligning tickmarks to an image. I've created a correlation matrix for 84 datasets. I'm visualizing the matrix as an image with colour coding according to the correlation coefficient.

The 84 datasets are distributed over three factors, but the desgin is unbalanced, so that the tickmarks and the lables for the axis must not evenly distributed. A regular grid via the 'grid' function aligns perfectly with the image cells, but the tickmarks via "axis" are slightely shifted, and not aligned perfectly with the image cells. The offset is even stonger for the y-axis. The thing is that I don't want 84 lables at the axis, it's enough to have one lable for all the different factor level combinations, which results in 28 labels.

Maybe you have an idea how to setup the command to align the tick marks properly.
	
	thanks for your help, kind regards,

	Arne


Here are my commands:

> library(marrayPlots) # for the colors
> col <- maPalette(low='white', high='darkred', k=50)
> par(ps=8, cex=1, mar=c(1,5,5,1)) # space needed for lables @ axis 1 and 3

# x and y range from 1 to 84, x is the correlation matrix (dim = 84x84)
> image(1:84, 1:84, x, col=col, xaxt='n', yaxt='n', xlab='', ylab='')

# set up the axis, 28 lables, distributed un-evenly over the image axis
> axis(3, i, labels=names(l), las=2, tick=T)
> axis(2, i, labels=names(l), las=2, tick=T)
> grid(84, col='black', lty='solid') # grids each of the 84 cells

# this is where the lables come form, the number indicate the replicates
# per factor-level combinations
> l
    NEW:4:0   NEW:4:100   NEW:4:250   NEW:4:500  NEW:4:1000    NEW:24:0 
          3           3           3           3           3           3 
 NEW:24:100  NEW:24:250  NEW:24:500 NEW:24:1000     OLD:4:0   OLD:4:100 
          3           3           3           3           4           3 
  OLD:4:250   OLD:4:500  OLD:4:1000    OLD:24:0  OLD:24:100  OLD:24:250 
          2           3           3           4           3           2 
 OLD:24:500 OLD:24:1000     PRG:4:0   PRG:4:100   PRG:4:250  PRG:4:1000 
          3           3           3           3           3           3 
   PRG:24:0  PRG:24:100  PRG:24:250 PRG:24:1000 
          3           3           3           3 

# these are the positions along the axis for the tick marks,
# replicates from 1 to 3 (replicates of one factor-evel combination), 4 to 6  
# ...
> i
 [1]  3  6  9 12 15 18 21 24 27 30 34 37 39 42 45 49 52 54 57 60 63 66 69 72 75
[26] 78 81 84