extra white pixel row in PNG

[Mojca M. <moj...@gm...>]

Hello,

Today I was trying to generate PNG "heatmaps" (similar to this one:
http://gnuplot.sourceforge.net/demo/pm3d.8.png) with one-to-one pixel
ratio, but I cannot get rid of the extra white row below data and one
extra white column on the right:

# remove everything but the plot
set tmargin 0
set bmargin 0
set lmargin 0
set rmargin 0
unset colorbox
unset border
unset xtics
unset ytics
unset key
set size ratio -1

n=4
# I have (x,y,z) tripples with x and y from (0,1,2,3,4)
# so that's a 5x5 matrix
set xrange [-0.5:n+0.5]
set yrange [-0.5:n+0.5]

# this doesn't work as desired, but leaves an extra blank row and column
set term png size n+1,n+1
plot "my-5-times-5.dat" matrix with image

At the end I solved the problem by using
set term png size n+2,n+2 crop
so that the crop command removed the extra line and column, but it
seems more like a tiny bug to me (most probably resulting from
fundamentally slightly different behaviour between pixel and vector
formats/different perception of whether one draws in the center of
pixels or on the border). Any comments or ideas about this?

Thanks,
    Mojca