set output 'cubic_a.ps' 
set term postscript color enhanced 'Helvetica,18'  
set style line 1 lt rgb 'red' lw 3  
set style line 2 lt rgb 'green' lw 3  
set style line 3 lt rgb 'blue' lw 3  
set style line 4 lt rgb 'cyan' lw 3  
set style line 5 lt rgb 'violet' lw 3  
set grid 
set key top right 
set ylabel 'Potential energy (J) '  
set xlabel 'Displacement (m) ' 
set title '' 
plot [-15:35] x*x*x - 30*x*x + 6*x - 20          with lines ls 1 lw 6 t ''  
set output 
quit 
