> with(plots):

Warning, the name changecoords has been redefined

> with(DEtools):

> DEplot([diff(x(t),t)=y(t),diff(y(t),t)=-9*x(t)+2*x(t)^3],[x(t),y(t)],-0.15..0.15,{[0,4,0],[0,-4,0],[0,6,0],[0,-6,0],[0,0,50],[0,0,-50],[0,0,50],[0,0,-50],[0,0,40],[0,0,-40]},color=y^2+9*x^2-x^4,arrows=THICK,stepsize=.1,linecolor=black,title=`Soft oscillator, color=2*energy`);