## 1.2.6 Arrows

The following plot styles allow arrows or lines to be drawn on graphs with positions dictated by a series of data points:

- `arrows_head`

- `arrows_nohead`

- `arrows_twohead`

The plot style of `arrows` is an alias for `arrows_head`. Each of these plot styles take four columns of data on two-dimensional plots – , , and – or six columns of data on three-dimensional plots with additional -coordinates. Each data point results in an arrow being drawn from the point to the point . The three plot styles differ in the kinds of arrows that they draw: `arrows_head` draws an arrow head on each arrow at the point ; `arrows_nohead` draws simple lines without arrow heads on either end; `arrows_twohead` draws arrow heads on both ends of each arrow.

A diagram of fluid flow around a vortex.

In this example we produce a velocity map of fluid circulating in a vortex. For simplicity, we assume that the fluid in the core of the vortex, at radii , is undergoing solid body rotation with velocity , and that the fluid outside this core is behaving as a free vortex with velocity . First of all, we use a simple python script to generate a data file with the four columns:
`from math import *` `for i in range(-19,20,2):` `for j in range(-19,20,2):` `x = float(i)/2` `y = float(j)/2` `r = sqrt(x**2 + y**2) / 4` `theta = atan2(y,x)` `if (r 1.0): v = 1.3*r` `else : v = 1.3/r` `vy = v * cos(theta)` `vx = v * -sin(theta)` `print "%7.3f %7.3f %7.3f %7.3f"%(x,y,vx,vy)`
This data can then be plotted using the following Pyxplot script:
`set size square` `set width 9` `set nokey` `set xlabel ’x’` `set ylabel ’y’` `set trange [0:2*pi]` `plot` `’vortex.dat’ u 1:2:($1+$3):($2+$4) w arrows,` `parametric 4*sin(t):4*cos(t) w lt 2 col black`