fitting a function with multiple data sets using gnuplot

Question

I would like to fit a function using many data sets. For example, I reproduce an experience many times, each time I obtain a pair of data column (x,y). I put all these column in a file named 'data.txt' :

first experience : x = column 1, y = column 2

second experience : x = column 3, y = column 4

third experience : x = column 5, y = column 6

...

Now I wish to fit a function y = f(x) for these data sets. I do not know if Gnuplot can do that ? If it is possible, could you please help me to correct the following command ? This one does not work.

fit f(x) "data.txt" u 1:2:(0.25), "data.txt" u 3:4:(0.25), "data.txt" u 5:6:(0.25) via a, b

Answer 1

You can process your data so that columns 1, 3 and 5 all become the same column 1, and columns 2, 4 and 6 all become the same column 2. It's easy with awk, you can do it outside gnuplot:

awk '{print $1, $2} {print $3, $4} {print $5, $6}' data.txt > data2.txt

and then fit it within gnuplot:

f(x)=a*x+b
fit f(x) "data2.txt" u 1:2:(0.25) via a,b

Or you can do it completely within gnuplot without any intermediate file:

f(x)=a*x+b
fit f(x) "< awk '{print $1, $2} {print $3, $4} {print $5, $6}' data.txt" u 1:2:(0.25) via a,b