set term qt font 'serif,14'
set xlabel "y"
set ylabel "x"

h(x) = 3*x**2


a = 0.7
d = 0.05
set arrow nohead from a, h(0) to a, h(a) ls 2
set arrow nohead from a+d, h(0) to a+d, h(a+d) ls 2
set arrow nohead from 0, h(a) to a, h(a) ls 2
set arrow nohead from 0, h(a+d) to a+d, h(a+d) ls 2


plot [0:1] h(x)