I have 2 dat files:
a.dat
#Xs
100 25
200 56
300 75
400 67
b.dat
#Xs
100 65
200 89
300 102
400 167
I want to draw a graph in the gnuplot where the yy values are a ratio between the values of a.dat and b.dat respectively. e.g., 25/65, 56/89, 75/102, and 67/167.
How I do this? I only know make a plot like this, and not with the ratio.
plot "a.dat" using 1:2 with linespoints notitle
"b.dat" using 1:2 with linespoints notitle
You cannot combine the data from two different files in a single using statement. You must combine the two files with an external tool.
The easiest way is to use paste:
plot '< paste a.dat b.dat' using 1:($2/$4) with linespoints
For a platform-independent solution you could use e.g. the following python script, which in this case does the same:
"""paste.py: merge lines of two files."""
import sys
if (len(sys.argv) < 3):
raise RuntimeError('Need two files')
with open(sys.argv[1]) as f1:
with open(sys.argv[2]) as f2:
for line in zip(f1, f2):
print line[0].strip()+' '+line[1],
And then call
plot '< python paste.py a.dat b.dat' using 1:($2/$4) w lp
(see also Gnuplot: plotting the maximum of two files)
The short answer is that... you cannot. Gnuplot processes one file at a time.
The work-around is to use an external tool, e.g. using the shell if you have a unix-like, or gnuplot.
join file1 file2 > merged_file will allow you to merge your files quite easily if the first column is identical in both files. Options allow to join on other columns and manage data missing in either file.
In case there is no common column but hte line number is relevant, paste will do.
In case interpolation is required, because the grids in the two files differ, you have to code this. I have a command-line utility that I distribute as free open-source software and can help: datamerge.
There is a trick if the two datasets don't fit (different sampling in x), but you have a good mathematical model for at least one of them:
fit f2(x) data2 us 1:2 via ...
set table $corr
plot data1 using 2:(f2($1))
unset table
plot $corr using 1:2
This is of course nonsense if both datasets have the same set of independent variables, because you can simply combine them (see other answers).
Just for fun, there is a gnuplot-only and platform-independent solution.
For sure, the command plot '< paste a.dat b.dat' using 1:($2/$4)' from Christoph's answer is unbeatable short (and certainly efficient) in case you are working under Linux or installed the CoreUtils from GnuWin under Windows.
The solution below takes the detour via two long strings. This should work fine unless there is a length limit of strings in gnuplot. I tested only until 100'000 data lines, which will take quite a few minutes. The assumption is that the two files have equal lines with identical x-values. For gnuplot>=5.0 you could write into a datablock instead of a file on disk and do further optimizations.
Script: (works with gnuplot>=4.6.0, March 2012)
### get ratio of numbers from different files
reset
FILE1 = "SO20069641a.dat"
FILE2 = "SO20069641b.dat"
FILE = "SO20069641.dat"
# create some random test data
set samples 100
set table FILE1
plot '+' u (int($0+1)*100):(int(rand(0)*100)+1)
set table FILE2
plot '+' u (int($0+1)*100):(int(rand(0)*100)+1)
unset table
a = ''
b = ''
stats FILE1 u (a=a.' '.strcol(1).' '.strcol(2)) nooutput
stats FILE2 u (b=b.' '.strcol(1).' '.strcol(2)) nooutput
set table "SO20069641.dat"
set samples words(a)/2
splot '+' u (n=2*(int($0+1)),real(word(a,n-1))):(real(word(a,n))):(real(word(b,n)))
unset table
plot FILE u 1:($2/$3) w impulses
### end of script
Result:
