How to display only specific tics with GNUPLOT

With the data set just below, I want to change the xtics displayed

$data << EOD
xxx   -2 -1 0 1 2
0     4  5  6 -3 -1
1     3  4  7 -4 -2
2     2  3  6 6  4 
3     4  -3 -6 4 1
EOD

With the simple command plot $data matrix columnheaders rowheaders with image , I got the image :

Now, if I add one xtick with set xtics add ("T" 0.577), I got the T label, but not at the right place (I would have expected to be between 0 & 1) as shown below :

Then, I do not manage to have only the T label without the other xtics, I tried the following sequence :

  unset xtics
  set xtics ("T" 0.577)
  replot

I still get all the xtics as defined in the original dataset.

Let me try to explain. If you plotted the matrix

x  A  B  C
K  1  2  3
L  4  5  6
M  7  8  9

the xtic labels would be A, B, C, since gnuplot takes the columnheaders and rowheaders as text, not as numbers.

So, if you plot a MxN matrix with image, the x- and y-coordinates will actually be from 0 to M-1 and from 0 to N-1, respectively. In your case, although, the xtic labels show -2 to 2 (i.e. these are just text labels) the x-range is actually from 0 to 4.

Hence, as a solution you could interpolate your desired value in the range 0 to N-1.

Maybe there are other possibilties, e.g. with splot and pm3d, but this would depend on your exact requirements.

In the example below stats (check help stats) is used to find the minimum and maxium x-coordinate and the number of columns of your matrix. You will get a warning warning: matrix contains missing or undefined values, because you have xxx as non-numerical value in your matrix. You can either ignore the warning or avoid the warning by replacing xxx, e.g. by 0.

Script:

### add special xtic on matrix with image
reset session

$Data << EOD
xxx  -2 -1  0  1  2
0     4  5  6 -3 -1
1     3  4  7 -4 -2
2     2  3  6  6  4
3     4 -3 -6  4  1
EOD

stats $Data matrix every ::1:0::0 u 3 nooutput
x0 = STATS_min
x1 = STATS_max
N  = STATS_columns-1
interpolate(x) = (x-x0)/(x1-x0)*N

set xtics out
set xtics add ("T" interpolate(0.577))
set xrange noextend

plot $Data matrix columnheaders rowheaders w image
### end of script

Result: