set xrange [-1:3]
set yrange [-1:4]
set zeroaxis
set xlabel 'x'
set ylabel 'f(x)'
set nokey
set label 1 'a' at 0.5,-0.1
set label 2 'b' at 2.5,-0.1
set label 3 ' M_1' at 0,1
set label 4 ' M_2' at 0,2.02
set noxtics 
set noytics 
f(x)=x<0.5 ? 1/0 : x>2.5 ? 1/0 : -(x-1)**2 +(x-1)**3+1  
g(x)=x<0.5 ? 1/0 : x>1.5 ? 1/0 : 1
z(x)=x<1.5 ? 1/0 : x>2.5 ? 1/0 : 2.02
plot f(x) with lines , g(x) with lines, g(x) with impulse, z(x) with lines, z(x) with impulse
set terminal postscript enhanced
set output "c:\\quattro.ps"
replot