-------------------------------------------------------------------------------
example.gnuplot:
# ..
set xtics rotate \
("Mar 10, 1999" 990310, \
"Mar 11, 1999" 990311, \
"Mar 12, 1999" 990312, \
"Mar 13, 1999" 990313)
# ..
-------------------------------------------------------------------------------
Another remark - I used Gnuplot 3.5 (pre 3.6) / patchlevel beta 340.
Regards,
Marc