reset
set border
set xtics 1
set ytics 5
set xzeroaxis lt 9 lw 5
set yzeroaxis lt 9 lw 5
set style line 1 lt 1 lw 4
set style line 2 lt 3 lw 4
set style line 3 lt 10 lw 4
set style line 4 lt 12 lw 4
set style line 5 lt 9 lw 1
set grid ls 5
show grid
set xrange [-6:9]
set yrange [-20:70]
# set samples 1000
set key on
f(x)=3*x-5/2
g(x)=-x**2+8*x
h(x)=2*x**3+18*x**2+54*x+55 
# i(x)=4*x**2+40*x+84
# plot f(x) ls 1
plot f(x) ls 1,g(x) ls 2, h(x) ls 3