# set terminal pngcairo  transparent enhanced font "arial,10" fontscale 1.0 size 500, 350 
# set output 'pointsize.1.png'
unset border
unset key
set noxtics
set noytics
set noztics
set title "plot with variable size points" 
plot 'world.dat' with lines lt 3,      'world.cor' using 1:2:(5.*rand(0)) with points lt 1 pt 6 ps variable