Welcome to the Surrey Physics Plotters blog. It exists in the spirit of other blogs dedicated to the use of the

`gnuplot`plotting program. While I don't consider myself as much of an expert at gnuplot as the writers of gnuplotting or gnuplot tricks, I think it would still be useful to have another repository for useful examples that I, at least, will otherwise forget.So - the first example comes in the form of a plot which includes a set of data defined at equally spaced points in the abscissa, whose ordinates show a sudden kink in the gradient (the data are the radii of a series of isotopes of lead nuclei - if you're interested, the paper is here).

I wanted to include on the plot a fit to the data, assuming one straight line up to atomic number 208, and a different straight line from 208 up. I'll show the code here, and explain how it works below.

The first two lines set up the output file as a postscript file (since this is what I wanted for the publication). Specifying the size here adds in the BoundingBox for encapsulated postscript, though it probably makes sense to use the eps option instead.setterm postscript enhanced col size 10cm,8cm font 'Helvetica,12'

setout 'lead-exp.eps'

f1(x) = m1*(x-208)

f2(x) = m2*(x-208)

fit[202:208] f1(x) 'expdat.gnu' using 1:($2-5.5010):3 via m1

fit[208:214] f2(x) 'expdat.gnu' using 1:($2-5.5010):3 via m2

f(x) = abs(x-205)<3 ? f1(x) : abs(x-211)<3 ? f2(x) : 1/0

setxrange [201:215]

setyrange [-0.05:0.08]

setylabel '{/Symbol d}<r_{ch}^2>^{1/2} [fm]'

setxlabel 'A'

plot'expdat.gnu' u 1:($2-5.5010):3 w yerr ls 1 notit, f(x) lt 2 notit

quit

# Data from I. Angeli, At. Data Nucl. Data Tables, 87, 185 (2004)

202 5.4690 .0055 .0007

204 5.4794 .0008 .0005

206 5.4897 .0007 .0003

208 5.5010 .0009

210 5.5230 .0035 .0005

212 5.5450 .0075 .0010

214 5.5650 .0105 .0014

The two separate fitting functions are defined as

`f1(x)`and

`f2(x)`. They have undetermined parameters

`m1`and

`m2`and are designed to be zero at

`x=208`.

I then fit the two separate lines in specified ranges, using the data that comes at the end of the plot file - I have assumed that the above file is saved as

`expdat.gnu`.

The line defining

`f(x)`makes a piecewise function, which is

`f1(x)`if

`x`is in the range

`202<x<208`, is

`f2(x)`if it is in the range

`208<x<214`and undefined otherwise - the "

`1/0`" serves to make the function undefined.

Everything else is a bit more straightforward, I think. The range in x and y are set, as are the labels, though the

`enhanced`option in the

`set term`line means that the

`_`and

`^`symbols are treated as subscript and superscript indicators as in LaTeX in the label text. The results (turned into a png file) is shown at the top.