Making a vector plot

When the quantity that depends on x and y has both magnitude and a direction it can be represented by an arrow whose length is proportional to the magnitude. The x-y plane can be broken into a regular grid and an arrow drawn in the plane, representing the direction and magnitude associated with each point on the grid—a vector plot. As we are associating two quantities, the magnitude and direction, or, equivalently, Δx and Δy, for each x-y pair, we can think of this type of plot as a 4D plot.

The previous figure is a vector field of a rotating flow. The exact appearance of the arrows will vary with the output terminal.

How to do it…

The following script draws the vector plot in the previous figure:

set xrange [0:pi]
set yrange [0:pi]
set iso 20
set samp 20
unset key
a = .2
plot '++' using 1:2:(-a*sin($1)*cos($2)):(a*cos($1)*sin($2))\
          w vec size  .06, 15 filled

How it works…

Vector plots require a data file or equivalent pseudofile. Four columns are used for x, y...