gnuplot: Version 5.0 patchlevel 1 last modified 2015-06-07
system: Kubuntu 16.10
For the first column of layout, negative X offset moves only right boundary of the plot, left boundary is unchanged. Y offset works fine. Positive offsets also work fine.
MWE:
set term png
set out "test.png"
set multiplot layout 2,2 offset -0.2
plot x
plot x
plot x
plot x