Possible solution

Content

Modified plot for comparing run times

set terminal svg size 900, 500
set output "initmatrix_sol.svg"
set xlabel "Matrix dim A: M=N"

# add label for yaxis
set ylabel "Wall time (in sec.)"

# log scale for both axis
set logscale x
set logscale y

# change font size of title
set title font ",24"
set title "matrix initialization"
set key outside
set pointsize 0.5
plot "initmatrix.data" using 1:3 with linespoints lt 2 lw 3 title "col-major", \
     "initmatrix.data" using 1:4 with linespoints lt 3 lw 3 title "row-major"

Generating a plot

heim$ gnuplot initmatrix_sol.gnuplot
heim$ 

Plot of speedup

set terminal svg size 900, 500
set output "initmatrix_sol2.svg"
set xlabel "Matrix dim A: M=N"

# add label for yaxis
set ylabel "t2/t1"

# change font size of title
set title font ",24"
set title "speedup row major vs col major"
set key outside
set pointsize 0.5
plot "initmatrix.data" using 1:5 with linespoints lt 2 lw 3
heim$ gnuplot initmatrix_sol2.gnuplot
heim$