Gnuplot can do curve fitting (of all kinds) and parks the coefficients in variables. In general, you’d like to display those values on the final plot for later reference…
The trick is using the sprintf() function, which behaves largely like the C version, to insert the variable into a formatted string for use in the label command.
I drive Gnuplot with shell scripts, which simplifies introducing parameters & suchlike. That’s conspicuous by its absence here, but when you need it, you need it bad.
The script to generate that plot looks like this, with some key points in the highlighted lines:
#!/bin/sh
export GDFONTPATH="/usr/share/fonts/truetype/msttcorefonts/"
gnuplot << EOF
set term png font "arialbd.ttf" 18 size 950,600
set output "Calibration Curve - Full.png"
set title "Calibration Curve - Full"
set key noautotitles
unset mouse
set bmargin 4
set grid xtics ytics
set xlabel "10^5/ADC"
set format x "%3.0f"
set ylabel "Resistance - Ohm"
set format y "%3.0f"
set yrange [0:100]
set datafile separator "\t"
f(x) = m*x + c
fit f(x) "Measurements/Calibration.csv" using 3:1 via m,c
set label 1 sprintf("m = %3.4f",m) at 510,75 font "arialbd,18"
set label 2 sprintf("c = %3.4f",c) at 510,70 font "arialbd,18"
plot \
"Measurements/Calibration.csv" \
using 3:1 with linespoints lt 3 lw 3 pt 3 , \
f(x) lt 4 lw 2
EOF
The dataset for that plot is tucked into the obvious file and looks like this, with tabs between the columns:
# ESR Calibration Curve # Resistance ADC Decimal Reciprocal 0.0 492 203 0.1 489 204 1.0 461 217 1.2 456 219 1.5 447 224 1.8 440 227 2.0 432 231 2.3 428 234 2.4 423 236 2.7 414 242 3.3 400 250 3.8 387 258 4.3 378 265 4.7 367 272 5.0 360 278 5.5 350 286 6.3 332 301 6.7 329 304 8.1 306 327 9.1 293 341 9.9 284 352 21.0 182 549 33.0 126 794 47.0 90 1111 67.0 60 1667 73.0 54 1852 83.0 47 2128 92.0 41 2439
There is no denying that a straight line is not the best fit to that dataset, but that’s not the point.
Memo to Self: the set label commands go between the fit and the plot. Remember to add the f(x) to the plot function…
The Smell of Molten Projects in the Morning 
Any idea how to get the values for the fitted line? I would have thought that set table would have done the trick, but that looks like a no, go.
Any help would be appreciated.
That’s the sprintf(“c = %3.4f”,c) part of the highlighted area: you define the coefficients in the f(x) = m*x + c line, then display them as labels exactly where you want them.
Not obvious, I grant you, but do-able!
Thanx man!! Have been searching for this for quite a while now! :) Cheers!!
It’s fairly well hidden in the doc, isn’t it?
Thanks Ed, this was a huge help for me to get the fit parameters into a label.
Glad to help!
It’s obvious once you see it, but until then …