Re: [Gnuplot-info] The use of $ in scripts and simple data transformations in scripts

[Thomas S. <t.s...@fz...>]

gnuplot manual or 'help call' says:

NOTE: there is a clash in syntax with the datafile using callback
operator. Use $$n or column(n) to access column n from a datafile
inside a called datafile plot.


Beyer wrote:
> 
> I have a data file where column one is the x-values. The second column is
> a measure of current and the third is a measure of the potential drop
> across a device.
> I need to plot the conductance of the device which means I have to take
> column 2 and divide it with column 3.
> Something like plot 'bla.dat' u 1:($2/$3)
> U = IR >> U/I = R >> I/U = 1/R = G
> 
> Now I am making a multiplot since I am comparing five different graphs and
> I am doing this by calling a script that works fine just plotting the
> current alone, for example. The problem comes when I try to follow the
> example given in "gnuplot in action" on page 41.
> that says: "If an argument to using is enclosed in parentheses, it’s not
> treated as a column number, but as an expression to be evaluated. Inside
> the parentheses,
> you can access the values of the column values for the current record by
> preceding
> the column number with a dollar sign ($) (as in shell or awk programming).
> Some examples will help to clarify.
> 
> ...
> 
> To plot the average of the second and third columns, use
> plot "data" using 1:( ($2+$3)/2 ) with lines"
> 
> MY PROBLEM IS:
> 
> CODESNIPPET FROM MY SCRIPT:
> plot '$3.dat' u 1:($2/$3) w l ls 1
> 
> now the example in the book seems to indicate I have to use the dollar
> sign in the parenthesis to indicate that I am talking about a column
> number in the expression but when gnuplot reads the script it quite
> naturally seems to read the two values as the fourth and third option fed
> to the script when I call it from gnuplot.
> This clearly isn't what I really want it to do, but I can't figure out how
> I tell gnuplot that the $3 means "column three" not "script option 4"
> -----------------
> I call the script by calling:
> call 'multi.gp' 'firstmultitest' '3' '2' 'datafile' 'datafile2'
> 'datafile3' 'datafile4' 'datafile5'
> 
> The script as it appears now looks like:
> #0=filename for the final graph 1=number of rows 2=number of columns
> 3=data filename 4=data filename 5=data filename 6=data filename 7=data
> filename 8=data filename
> set terminal push
> set output "$0.png"
> set style line 1 lt -1 lw 2
> set terminal png size 1000,600 font Arial 16
> set multiplot layout $1,$2
> unset key
> unset tics
> set ytics 0,0.1 nomirror
> set xtics -10,2.5 nomirror
> unset border
> set border 3 lt -1 lw 2
> plot '$3.dat' u 1:($2/$3) w l ls 1
> plot '$4.dat' u 1:(2/3) w l ls 1
> plot '$5.dat' u 1:(2/3) w l ls 1
> plot '$6.dat' u 1:(2/3) w l ls 1
> plot '$7.dat' u 1:(2/3) w l ls 1
> #plot '$8.dat' u 1:(3) w l ls 1 #not needed yet
> unset multiplot
> replot
> set output
> set terminal pop
> 
> 
> Thank you very much for your help.
> 
> 

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