8.27.1 arrow

set arrow <arrow number> from [<co-ordinate>] <x>,
          [<co-ordinate>] <y> to [<co-ordinate>] <x>,
          [<co-ordinate>] <y> [with <modifier> ]
<co-ordinate> = ( first | second | screen | graph |
                  axis<axisnumber>                  )

The set arrow command causes an arrow to be added to a plot. An example of its syntax would be:

set arrow 1 from 0,0 to 1,1

which would cause an arrow to be drawn between the points $(0,0)$ and $(1,1)$, as measured on the $x$ and $y$ axes. The tag 1 immediately following the arrow keyword is an identification number, and allows the arrow to be removed later with the unset arrow command. By default the co-ordinates are measured relative to the first $x$- and $y$-axes, but can be specified in a range of co-ordinate systems. These are specified as follows:

set arrow 1 from first 0, second 0 to axis3 1, axis4 1

As can be seen, the name of the desired co-ordinate system precedes the position value in that co-ordinate system. The co-ordinate system first, the default, measures the graph using the $x$- and $y$-axes. second uses the $x2$- and $y2$-axes. screen and graph both measure in centimetres from the origin of the graph. The syntax axisn may also be used, to use the $n$ th $x$- or $y$-axis; for example, axis3 above.

The set arrow command can be followed by the keyword with, to specify the style of the arrow. For example, the specifiers nohead, head and twohead, after the keyword with, can be used to make arrows with no arrow heads, normal arrow heads, or two arrow heads. twoway is an alias for twohead. Normal line type modifiers can also be used here. For example:

set arrow 2 from first 0, second 2.5 to axis3 0,
             axis4 2.5 with colour blue nohead