3.4.1 Arrows

Arrows may be placed on plots using the set arrow command, which has similar syntax to that used by gnuplot. A simple example would be:

set arrow 1 from 0,0 to 1,1

The number `1' immediately following `set arrow' specifies an identification number for the arrow, allowing it to be subsequently removed via:

unset arrow 1

or equivalently, via:

set noarrow 1

In PyXPlot, this syntax is extended; 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'. For example:

set arrow 1 from 0,0 to 1,1 with nohead

In addition, linestyles and colours can be specified after the keyword `with':

set arrow 1 from 0,0 to 1,1 with nohead \
linetype 1 c blue

As in gnuplot, the coordinates for the start and end points of the arrow can be specified in a range of coordinate systems. `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. In the following example, we use these specifiers, and specify coordinates using variables rather than doing so explicitly:

x0 = 0.0
y0 = 0.0
x1 = 1.0
y1 = 1.0
set arrow 1 from first  x0, first  x1 \
            to   screen x1, screen x1 \
            with nohead

In addition to these four options, which are those available in gnuplot, the syntax `axisn' may also be used, to use the $n$th $x$- or $y$-axis - for example, `axis3'. This allows arrows to reference any arbitrary axis on plots which make use of large numbers of parallel axes (see section 3.3.1).