Gnuplot "vector line"

Question

I am trying to generate a plot which uses arrows as markers in Gnuplot. These arrows I want to turn in a specific angle which I know. So I have value triples of x1 ... xn, y1...yn, alpha1...alphan. Sorry, I wasn't able to include a pic from my hard drive to illustrate what I want to achieve.

Basically, for every (15th or so) x-y pair, the marker should be an arrow which uses a certain angle.

The measured data is tightly packed so I suppose I will have to define an increment between the markers. The length of the arrow can be the same all over.

I would appreciate your ideas.

Answer 1

Gnuplot has a plot mode with vectors that is what you want

Given that your file has the following format, x y angle and assuming that your angle is in radians, you have to take into account that

1. with vectors requires 4 parameters, namely x y dx dy where dx and dy are the projections of the lenght of the arrow.

2. this draws only the arrows, if you want a line you have to make two passes on the data.

3. you want to draw an arrow for a data point over, say, 10 points.

That said, I'd proceed like this

dx(a) = 0.2*cos(a) # 0.2 is an arbitrary scaling factor
dy(a) = 0.2*sin(a)
# this draws the arrows
plot 'mydata.dat' every 10 using 1:2:(dx(a)):(dy(a)) with vectors
# this draws the line
plot 'mydata.dat'

You may want to use help plot to find the detailed explanation of all the parameters that you can apply to a with vectors plot.

Credits: An article on the gnuplotting site