Here is a test script for gnuplot:

reset
set encoding utf8
set terminal pngcairo enhanced
set grid
set xlabel 'α'
set ylabel 'Δβ'
set output "./test.png"
plot [x=0:5] exp(-x)*cos(5*x) with lines title "I_{b} = 0,2 Ампер"

When I run this from command line with gnuplot test.gpi, I get the following output:

But when I open the same file in emacs and issue the command gnuplot-send-buffer-to-gnuplot, I get: