histograms are shifted to the right when gap size is an even number, everything's ok for odd gaps.
i first thought of an integer division, which is really there in 'graphics.c':
dxl += histogram_opts.gap/2;
dxr += histogram_opts.gap/2;
BUT with '2.0' instead of '2' as divisor ALL histograms are shifted to the right by half a box width, so in addition i subtract half a box size and now all histograms look ok.
i attach the patch plus a script with datafile to show the error.
nearly the same code is also in 'plot_bars()' in 'graphics.c' but i could figure out when it is used... so i didn't correct this part.