The code is:
jsxgraph // boundingbox:[left, top, right, bottom] var brd = JXG.JSXGraph.initBoard(divid, {axis: true, boundingbox: [-0.5, 4, 5, -0.5], showCopyright: false, shownavigation: false}); var xaxis = brd.create('text', [4.5,-0.3, "U"]); var yaxis = brd.create('text', [-0.4,3.5, "V"]); // create nullcline dummies var l1p1 = brd.create('point',[4,2], {name:'',face:'<>',strokeColor:'green',size:2}); var l1p2 = brd.create('point',[4,2.5], {name:'',face:'<>',strokeColor:'green',size:2}); var l1 = brd.create('line',[l1p1,l1p2], {straightFirst:false, straightLast:false,strokeColor:'green',strokeWidth:2,dash:1}); var l2p1 = brd.create('point',[4.5,2], {name:'',face:'<>',strokeColor:'green',size:2}); var l2p2 = brd.create('point',[4.5,2.5], {name:'',face:'<>',strokeColor:'green',size:2}); var l2 = brd.create('line',[l2p1,l2p2], {straightFirst:false, straightLast:false,strokeColor:'green',strokeWidth:2,dash:1}); var l3p1 = brd.create('point',[4,1], {name:'',face:'<>',strokeColor:'green',size:2}); var l3p2 = brd.create('point',[4,1.5], {name:'',face:'<>',strokeColor:'green',size:2}); var l3 = brd.create('line',[l3p1,l3p2], {straightFirst:false, straightLast:false,strokeColor:'green',strokeWidth:2,dash:1}); var l4p1 = brd.create('point',[4.5,1], {name:'',face:'<>',strokeColor:'green',size:2}); var l4p2 = brd.create('point',[4.5,1.5], {name:'',face:'<>',strokeColor:'green',size:2}); var l4 = brd.create('line',[l4p1,l4p2], {straightFirst:false, straightLast:false,strokeColor:'green',strokeWidth:2,dash:1}); var button1 = brd.create('button', [3.5, 3.5, 'Calculate trajectory', function() {clearturtle(); run();}], {}); var ic = brd.create('point',[1,1],{size:4,name:'(U(0), V(0))'}); var UV = brd.create('turtle',[1,2],{strokeColor:'blue',strokeWidth:3}); var b = 3; var a = 4; var t = 0; var U = ic.X(); var V = ic.Y(); var delta = 0.01; UV.hideTurtle(); function clearturtle() { UV.cs(); UV.hideTurtle(); } function run() { UV.setPos(ic.X(),ic.Y()); U = ic.X(); V = ic.Y(); t = 0; loop(); } function loop() { // define RHSs of the equation var dU = delta*(2*U*(1-U/b)-U*V); var dV = delta*(U*V-a*V); U = U+dU; V = V+dV; UV.moveTo([U,V]); t += delta; if (t<100.0) { active = setTimeout(loop,10); } } brd.unsuspendUpdate(); /jsxgraph