{first | second | graph | screen} x,\ {first | second | graph | screen} y\ {, {first | second | graph | screen} z}
first places the x, y, or z co-ordinate 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—0,0 is bottom left and 1,1 is top right.
If the co-ordinate system is not specified, first is used.
If one (or more) axis is timeseries, the appropriate co-ordinate should be given as a quoted time string according to the timefmt format string. See set {x,y,z}data and set timefmt.