Uellue's Blog

Include a numeric variable in a Gnuplot title or legend

I was fitting a function to some data using Gnuplot, and then I wanted to include the result of the fit in the legend of my plot. Well, up to here the script looks like the following:

# my function to fit the data
f(x) = A*x**B

# initial values for the fit
A = 1
B = 1

# do the fit, use column 2 as x and the absolute value of column 5 as y
fit f(x) "data.csv" using 2:(abs($5)) via A, B
[... lots of output from fit ...]

Now the variables A and B have a new value, resulting from the fit:

print A, B
46.437770413374 0.153260265112221

I can plot the data together with the fit:

set output "data.png"
set terminal png

set logscale xy
set samples 10000

plot f(x), "data.csv" using 2:(abs($5))

This generates the following image:

Plot of the data, default legend

Now I would like to have a legend for the fit like the following:

"y=A*x**B, A=46.437; B=0.153"

After digging a bit in the documentation, I discovered that Gnuplot has an sprintf function that can be used more or less everywhere where a string literal would work. So that's how I got what I wanted:

plot f(x) title sprintf("y=A*x**B, A=%.3f; B=%.3f", A, B) ,\
"data.csv" using 2:(abs($5))

And that's what it looks like:

Plot of the data, legend including fit variables calculated earlier

Well, in the next steps the legend could be moved to the lower right corner, but I'll leave that as an exercise for the reader ;-)