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

f(x) = -b * x

fit f(x) 'data2' u 1:3 via b

set xlabel 'T, deg'
set ylabel 'q, мН/м'
plot 'data2' using 1:3:2:4 w xyerrorbars title 'q-plot', f(x) title 'fitting line'

pause -1