Plotting arrows with gnuplot - gnuplot

Question

I have data generated in a simulation. The generated data file looks something like this:
1990/01/01 99
1990/01/02 92.7
1990/01/03 100.3
1990/01/04 44.2
1990/01/05 71.23
...
2100/01/01 98.25
I can create a chart (trivially), by simply issuing the (long versioned) command:
plot "simulation.dat" using 1:2 with line
I want to add a third column which will add arrow information. The encoding for the third column would be as follows:
0 => no arrow to be drawn for that x axis value
1 => an UPWARD pointing arrow to be drawn for the x axis value
2 => a DOWNWARD arrow to be drawn for the x axis value
I am just starting to learn gnuplot, and will appreciate help in how I can use gnuplot to create the arrows on the first plot?

Answer 1

I dont think there is an automatic way to create all your arrows at the same time based on the third column. You will have to execute the following for each arrow that you want:
set arrow xval1,yval1 to xval2,yval2
You can also use relative arrows
set arrow xval1,yval1 rto 1,0
This will draw a horizontal arrow from xval1,yval1 to (xval1+1),yval1
There are plenty of options associated with the set arrow command:

Answer 2

If you didn't want the arrow head then you might try the impulses style (with impulses rather than with lines)
(If you still want the lines on top then you can plot twice).
If you really want the arrow heads then the following might help: It uses a for loop (or sorts) to add vertical arrows to a plot.
Gnuplot script, for loop within or adding to existing plot
Specifically:
create a file simloop.gp which looks like the following:
count = count+1
#save the count to count.gp
system 'echo '.count.' > count.gp'
#load the simloop shell
system "./simloop.sh"
#draw the arrow
load 'draw_arrow.gp'
if(count<max) reread
Then create a simloop.sh file that looks something like so
#!/bin/bash
#read the count
count=$(awk -F, '{print $1}' count.gp)
#read the file
xcoord=$(awk -v count=$count -F, 'BEGIN{FS=" ";}{ if(NR==count) print $1}' simulation.dat)
ycoord=$(awk -v count=$count -F, 'BEGIN{FS=" "}{ if(NR==count) print $2}' simulation.dat)
dir=$(awk -v count=$count -F, 'BEGIN{FS=" "}{ if(NR==count) print $3}' simulation.dat)
#choose the direction of the arrow
if [ \"$dir\" == \"0\" ]; then
echo '' > draw_arrow.gp
fi
if [ \"$dir\" == \"1\" ]; then
echo 'set arrow from ' $xcoord' ,0 to '$xcoord','$ycoord' head' > draw_arrow.gp
fi
if [ \"$dir\" == \"2\" ]; then
echo 'set arrow from '$xcoord',0 to '$xcoord','$ycoord' backhead' > draw_arrow.gp
fi
Then create a simulation.gp file that looks something like so:
count = 0;
max = 5;
load "simloop.gp"
set yrange[0:*]
plot "simulation.dat" u 1:2 w l
Make sure the shell file has executable permissions (chmod +wrx simloop.sh), load up gnuplot and type
load "./simulation.gp"
This worked for me with the data file
1 99 0
2 92.7 1
3 100.3 2
4 44.2 0
5 71.23 1
(For testing I got rid of the time formatting You should be able to put it back without too much trouble.)
Then I got this graph:
Which I think is more or less what you want.

Answer 3

Although the question is quite old, here is my answer.
One can use the vectors plotting style, which can use variable arrowstyles based on a column's value:
set style arrow 1 backhead
set style arrow 2 head
set yrange[0:*]
set xdata time
set timefmt "%Y/%m/%d"
plot "simulation.dat" using 1:2 with line,\
"" using 1:2:(0):(-$2):($3 == 0 ? 1/0 : $3) with vectors arrowstyle variable
It the value of a column is 1/0, the point is considered as undefined and is skipped.