## 8.2.6 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.

**Example: 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`