I have this plot
And as you can see, there\'s this gap between the zero and the bottom of the plot, is
I can't really test it without your data, but did you try to set the limits on the axis? if not set manually, octave sometimes takes liberties to shift the data.
https://www.gnu.org/software/octave/doc/interpreter/Axis-Configuration.html
Yes, it is possible. Use:
set xyplane at 0
to remove the offset.
On older gnuplot, you may need to use:
set ticslevel 0
instead.