set terminal pngcairo size 500, 500 enhanced font 'Verdana, 10'

f(x) = a + b * x

fit f(x) 'data' u 1:3 via a, b

set xlabel 'T, deg'
set ylabel 'sigma, мН/м^2'
plot 'data' using 1:3:2:4 w xyerrorbars title 'sigma-plot', f(x) title 'fitting line'

pause -1