Even simpler function plotter

The underlying JavaScript code

<input type="text" id="input" value="sin(x)*x">
<input type="button" value="plot" onClick="plotter()">
<input type="button" value="clear all" onClick="clearAll()"> 
<input type="button" value="add tangent" onClick="addTangent()">
<input type="button" value="add Derivative" onClick="addDerivative()">

var board = JXG.JSXGraph.initBoard('jxgbox', {boundingbox:[-5,8,8,-5], axis:true});
var f, curve; // global objects

function plotter() {
  var txt = JXG.GeonextParser.geonext2JS(document.getElementById('input').value);
  f = new Function('x','var y = '+txt+'; return y;');
  curve = board.create('functiongraph',[f,
                function(){ 
                  var c = new JXG.Coords(JXG.COORDS_BY_SCREEN,[0,0],board);
                  return c.usrCoords[1];
                },
                function(){ 
                  var c = new JXG.Coords(JXG.COORDS_BY_SCREEN,[board.canvasWidth,0],board);
                  return c.usrCoords[1];
                }]);
}

function clearAll() {
    JXG.JSXGraph.freeBoard(board);
    board = JXG.JSXGraph.initBoard('jxgbox', {boundingbox:[-5,8,8,-5], axis:true});
    f = null;
    curve = null;
}

function addTangent() {
    if (JXG.isFunction(f)) {
        board.suspendUpdate();
        var p = board.create('glider',[1,0,curve], {name:'drag me'});
        board.create('tangent',[p], {name:'drag me'});
        board.unsuspendUpdate();
    }
}

function addDerivative() {
    if (JXG.isFunction(f)) {
        board.create('functiongraph',[JXG.Math.Numerics.D(f),
                function(){ 
                  var c = new JXG.Coords(JXG.COORDS_BY_SCREEN,[0,0],board);
                  return c.usrCoords[1];
                },
                function(){ 
                  var c = new JXG.Coords(JXG.COORDS_BY_SCREEN,[board.canvasWidth,0],board);
                  return c.usrCoords[1];
                }], {dash:2});
    }
}