Gnuplot - Unable to properly plot two matrices with different sizes

My data file is as follows:

#smaller matrix
5   0.00    0.25    0.50    0.75    1.00
0.00    1   2   3   4   5
0.25    5   4   3   2   1
0.50    1   2   3   4   5
0.75    5   4   3   2   1
1.00    1   2   3   4   5


#bigger matrix
8   0.00    0.25    0.50    0.75    1.00    1.25    1.50    1.75
0.00     1  2   3   4   5   6   7   8
0.25    5   4   3   2   1   0   1   2
0.50    1   2   3   4   5   6   7   8
0.75    5   4   3   2   1   0   1   2
1.00    1   2   3   4   5   6   7   8
1.25    5   4   3   2   1   0   1   2
1.50    1   2   3   4   5   6   7   8
1.75    5   4   3   2   1   0   1   2

The gnuplot script is as follows:

set term png size 900, 400
set output 'matrices-test.png'
set multiplot layout 1,2

set xrange [0:1.0]
set yrange [0:1.0]
plot 'matrices-test' index 0 nonuniform matrix w image

set xrange [0:1.75]
set yrange [0:1.75]
plot 'matrices-test' index 1 nonuniform matrix w image

unset multiplot

When I run this script, the smaller matrix comes out distorted. The image is tilted and the data seems mangled:

If I put each matrix into a separate file, they are both plotted without problems.

Am I doing something wrong? Or is this a bug in gnuplot? (I'm using 5.4.1, the newest version, in Ubuntu Linux.)

Solution

You have hit a program bug. Your script works correctly in the development version (gnuplot 5.5) and the fix will be included in the next 5.4 stable release.

Work-around:

The bug has something to do with there being multiple data blocks in the input file. It is not triggered if the matrices are in separate files or if they are loaded into separate data blocks and plotted from there. So one work-around would be to select out the first data block externally:

unset key
set auto noextend

set multiplot layout 1,2

plot '<head --lines=7 matrices.dat' index 0 nonuniform matrix w image
plot 'matrices.dat' index 1 nonuniform matrix w image

unset multiplot