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The commands arrow, key, and 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 ticslevel); and `screen` specifies the screen area (the entire area—not just the portion selected by 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 xdata and timefmt. `gnuplot` will also accept an integer expression, which will be interpreted as seconds from 1 January 2000.