How to plot simple line segments in GNUPlot?

Question

I am trying to draw a rudimentary line segment from (0,0) to (0,1). I already have an input file, but I want to add a line to it.

I have read through Line plot in GnuPlot where line width is a third column in my data file? and Plot line segments from a datafile with gnuplot among many other examples. Everyone is doing something much more complex than what I want, I only want the line segment added to my GNUPlot script.

I normally enter the data in to GNUPlot thus:

$DATA << EOD
.... other data
EOD
$LS << EOL
0   0
0   1
EOL
plot $DATA using 1:2:3 with points
plot $LS with lines

but this doesn't work, nor does

plot $LS using 1:2 with lines

How can I plot this simple line segment from (0,0) to (0,1)?

Answer 1

You can use headless arrows to plot arbitrary line segments.

set xrange [0:1]
set yrange [0:1]
set arrow 1 from 0,0 to 0,1 nohead lw 3 lc 2
set arrow 2 from 0,0 to 1,1 nohead lw 2
plot NaN t''

Answer 2

What does "doesn't work" mean? You don't even show the resulting graph. By the way what do you use the 3rd column for?

Your first example will make a plot only with your data and then a new plot with only the line segment. And in your second example, the line from 0,0 to 0,1 in your plot is identical with the y-axis and therefore hard to see as long as the xrange starts from 0. You can easily check this, e.g. if you set the color to red or linewidth to 3 , e.g.

plot $LS u 1:2 lc "red" lw 3

You also can make your line segment "visible" if you set xrange[-0.2:]. Check the following example

Code:

### plotting simple line segments
reset session

$DATA << EOD
0.1  0.2  1
0.3  0.4  2
0.5  0.6  3
0.7  0.5  4
EOD

$LS1 << EOL
0   0
0   1
EOL

$LS2 <<EOL
0   0
1   1
EOL

set xrange [-0.2:]

plot $DATA using 1:2:3 with points pt 7, \
     $LS1 u 1:2 with lines, \
     $LS2 u 1:2 with lines
### end of code

Result: