The commands set arrow, set key, and set label allow you to draw
something at an arbitrary position on the graph. This position is specified
by the syntax:
{<system>} <x>, {<system>} <y> {,{<system>} <z>}
Each <system> can either be first, second, graph or screen.
first places the x, y, or z coordinate in the system defined by the left
and bottom axes; second places it in the system defined by the second axes
(top and right); graph specifies the area within the axes--0,0 is bottom
left and 1,1 is top right (for splot, 0,0,0 is bottom left of plotting area;
use negative z to get to the base--see set ticslevel); and screen
specifies the screen area (the entire area--not just the portion selected by
set size), with 0,0 at bottom left and 1,1 at top right.
If the coordinate system for x is not specified, first is used. If the
system for y is not specified, the one used for x is adopted.
If one (or more) axis is timeseries, the appropriate coordinate should
be given as a quoted time string according to the timefmt format string.
See set xdata and set timefmt. gnuplot will also accept an integer
expression, which will be interpreted as seconds from 1 January 2000.