   x(1)=3
   x(2)=0
   x(3)=0.3
   x(4)=0.2
   t=0
   h=0.01
   
   gset xrange [-1:4]
   gset yrange [-1:2]
   gset xtics -1,1,4
   gset ytics -1,1,2
   gset size ratio 0.6
   gset multiplot
   gset grid
   
   plot(x(1),x(2),'*')
   for istep=1:10000
     [x,t]=rk(x,t,h);
     if(rem(istep,20) == 0)
       plot(x(1),x(2),'*')
     end
   end
