## 1.52.1 arrow

    set arrow <arrow number>
      from [ <system> ] <x>, [ <system> ] <y>, [ [ <system> ] <z> ]
      to   [ <system> ] <x>, [ <system> ] <y>, [ [ <system> ] <z> ]
      [ with { <option> } ]

where `<system>` may take any of the values
`( first | second | screen | graph | axis<number> )`

The `set arrow` command is used to add arrows to graphs. The example

    set arrow 1 from 0,0 to 1,1

would draw an arrow between the points and , as measured along the `x` and `y`-axes. The tag `1` immediately following the keyword `arrow` is an identification number and allows arrows to be subsequently removed using the `unset arrow` command. By default, the coordinates are specified relative to the first horizontal and vertical axes, but they can alternatively be specified any one of several of coordinate systems. The coordinate system to be used is specified as in the example:

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

The name of the coordinate system to be used precedes the position value in that system. The coordinate 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 centimeters from the origin of the graph. The syntax `axis<n>` may also be used, to use the th horizontal or vertical 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`, when placed 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`. All of the line type modifiers accepted by the `plot` command can also be used here, as in the example:

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