gnuplot: internal error : STRING operator applied to undefined or non-STRING variable-CodePudding

I have multiple .txt files of the naming order

rp_sub-pilote3b_Phase1_Basis_a.txt rp_sub-pilote3b_Phase1_Basis_b.txt
rp_sub-pilote3b_Phase1_Basis_a.txt rp_sub-pilote3b_Phase1_Basis_b.txt
rp_sub-pilote3b_Phase1_Basis_a.txt rp_sub-pilote3b_Phase1_Basis_b.txt
rp_sub-pilote3b_Phase1_Basis_a.txt rp_sub-pilote3b_Phase1_Basis_b.txt

I want to create 4 plots corresponding to each phase. In each of these, I want to plot the first 3 columns from *_Basis_a.txt and *_Basis_b.txt corresponding to that phase. I have the following code, but it produces the error line 20: internal error : STRING operator applied to undefined or non-STRING variable

#load this this with  "./gnuplot.txt"
#set terminal qt enhanced 40


set terminal postscript enhanced color solid  "Helvetica" 10
set out "motion_translation.ps"
set key left bottom # for Position of Legend

set title "Motion across timesteps"   font "Helvetica,10"
set ylabel "displacement in mm"  font "Helvetica,10"
set xlabel "|time in TR"  font "Helvetica,10"
set size ratio 0.6

do for [n=1:4]{
    plot    sprintf("rp_sub-pilote3b_Phase.%d._Basis_a.txt", n) using 0:1  with lines lw 3  title "Not nulled x"  linecolor rgb "green" ,\
        sprintf("rp_sub-pilote3b_Phase.%d._Basis_a.txt", n) using 0:2  with lines lw 3 title "Not nulled y"  linecolor rgb "red" ,\
        sprintf("rp_sub-pilote3b_Phase.%d._Basis_a.txt", n) using 0:3  with lines lw 3 title "Not nulled z"  linecolor rgb "brown" ,\
        sprintf("rp_sub-pilote3b_Phase.%d._Basis_b.txt", n) using 0:1  with lines lw 3 title "nulled x"  linecolor rgb "turquoise" ,\
        sprintf("rp_sub-pilote3b_Phase.%d._Basis_b.txt", n) using 0:2  with lines lw 3 title "nulled y"  linecolor rgb "pink" ,\
        sprintf("rp_sub-pilote3b_Phase.%d._Basis_b.txt", n) using 0:3  with lines lw 3 title "nulled z"  linecolor rgb "black"
}

CodePudding user response：

Can't reproduce your problem exactly, my gnuplot version 5.4.1 shows another error message:

line 20: duplicated or contradicting arguments in plot options

This can be solved by putting the arguments for plot in the correct order - title before with:

... using 0:2 title "Not nulled y" with lines lw 3 linecolor rgb "red"