gnuplot development

Offical documentation says "`mirror` tells `gnuplot` to
put unlabelled tics at the same positions on the opposite
border."

However, "set xtics axis mirror" causes tics to be mirrored
on the opposite side of the x-axis, instead of above the
top border, as I had hoped.

I didn't file this bug under "documentation", since I'm
hoping the dev team will make it possible to mirror tics on
the *outside* of the border. Tics inside the borders are
covered by the data -i.e. not visible =(. I do not believe
it is currently possible to have tics on both top and
bottom, both on outside of the borders.

BTW, a decade+ of awesome work guys! Congrats!