Scripting with gnuplot--where sed - bash

Question

So I'm looking for some quick-and-dirty solution.
The problem:
I am trying to plot a specific section of a data file with gnuplot. This is fine. The basic line goes something like
plot "<(sed -n '1,100p' pointsandstuff.dat)" u 1:log($4**2+$5**2) notitle
This works just fine. The next step I want is to include in my title another part of the data, namely the data entry $3 (which for the points listed is identical, so I can parse it from anywhere). I run into problem because, while plot seems fine, I can't seem to feed regex info into 'title'. An example of something that doesn't work"
plot "<(sed -n '1,100p' pointsandstuff.dat)" u 1:log($4**2+$5**2) title "<(sed -n '1,1p' pointsandstuff.dat)"
(This would spit out a whole data line, in theory, though in practice I just get the title "<(sed...")
I tried attacking this with a bash script, but the '$'s that I use throw the bash script into a tizzy:
#!/bin/bash
STRING=$(echo|sed -n '25001,25001p' pointsandstuff.dat)
echo $STRING
gnuplot -persist << EOF
set xrange[:] noreverse nowriteback
set yrange[:] noreverse nowriteback
eval "plot "<(sed -n '25001,30000p' pointsandstuff.dat)" u 1:log($4**2+$5**2) title $STRING
EOF
Bash won't know what to do with '$4' and '$5'.

Answer 1

You seem to be attempting process substitution, but the double quotes stop it working in the first case and you need a command substitution in the second case.
You have:
plot "<(sed -n '1,100p' pointsandstuff.dat)" u 1:log($4**2+$5**2) \
title "<(sed -n '1,1p' pointsandstuff.dat)"
You need:
plot <(sed -n '1,100p' pointsandstuff.dat) u 1:log($4**2+$5**2) \
title "$(sed -n '1,1p' pointsandstuff.dat)"
The double quotes in the second case might not be strictly necessary, but you won't go wrong with them present.
Process substitution generates a file name and feeds the output of the nested command into that file; the command thinks it is reading a file (because it is reading a file).
Command substitution captures the output of the nested command in a string and passes that string to the command (when it is used as an argument to a command, as here).

Answer 2

My understanding of the question is a little hazy, but it looks like you want to plot the first 100 lines -- This is quite easy to do:
plot '< head -100 datafile.dat' u ....
Of course, you can use sed if you wish (or awk or ...). A gnuplot only solution might look like this:
plot 'datafile.dat' u ($0 > 100? 1/0:$1):(log($4**2+$5**2))
Or like this (which is more simple for regular selections):
plot 'datafile.dat' every ::25001::30000 u 1:(log($4**2+$5**2)
and explained in more detail in another answer.
Now, if you want the title to come from the datafile, you can parse it out using gnuplot's backtic substitution:
plot ... title "`head -1 datafile.dat | awk '{print $3}'`"
which is essentially the same as gnuplot's system command:
plot ... title system("head -1 datafile.dat | awk '{print $3}'")
but in this case, you might be able to use the columnhead function:
plot ... title columnhead(3)

Answer 3

Aha, thanks all. Have come up with a few solutions by now--the simplest being just escaping those $s from before (which I mistakenly thought gnuplot disliked...). To whit:
STRING=$(echo|sed -n '1,1p' spointsandstuff.dat)
echo $STRING
gnuplot -persist << EOF
set xrange[:] noreverse nowriteback
set yrange[:] noreverse nowriteback
eval "plot "<(sed -n '1,100p' pointsandstuff.dat)" u 1:(log(\$4**2+\$5**2)) title '$STRING'
!gv diag_spec.eps &
EOF
Thanks all, though--it's been a good excuse to play with this stuff...here's hoping that, if any poor soul sees this script later, it might be a bit easier on them.

Answer 4

Just for the records, instead of $4 or $5 you could write column(4) and column(5) and you don't have to think about escaping $ or not.
Furthermore, there is no need for sed, awk, head or other system calls which make the script platform dependent.
Here is a platform-independent gnuplot-only solution:
OP does not show example data, but what I understood from the description that the part of the data which should be plotted contains an identical text in column 3.
Note that row index is zero-based in gnuplot. The construct '+' u ... every ::0::0 is just one way of many ways to plot a single data point, and here it is just used for the title which has been extracted in the preceding plot command.
By the way, if OP wanted to plot only data which is defined by the text in column 3 (which is not clear from the question), e.g. in the example below PartB data, there would be another way, where you even don't have to specify the row indices M and N because gnuplot could simply filter the PartB data.
Data: SO12223772.dat
1 2 PartA 11 12
2 2 PartA 21 22
3 1 PartA 31 32
4 2 PartA 41 42
5 2 PartA 51 52
6 2 PartA 61 62
7 2 PartB 71 72
8 2 PartB 81 82
9 2 PartB 91 92
10 2 PartB 101 102
11 2 PartB 111 112
12 2 PartC 121 122
13 2 PartC 131 132
14 2 PartC 141 142
Script: (works with gnuplot>=4.4.0, March 2010)
### plotting only a part of data and extracting title from a column
reset
FILE = "SO12223772.dat"
M = 6 # row index 0-based
N = 10
set style line 1 pt 7 lc rgb "blue"
set key top left
f(col1,col2) = log(column(col1)**2+column(col2)**2)
plot FILE u 1:(f(4,5)) w lp ls 1 lc rgb "grey" ti "full data", \
'' u 1:(myTitle=strcol(3),f(4,5)) every ::M::N w lp ls 1 notitle, \
'+' u 1:(1/0) every ::0::0 w lp ls 1 ti myTitle
### end of script
Result: