View source for Change Equation of a Graph

This example shows how you can change the equation of a graph without creating the whole construction again.
<html>
<script type="text/javascript" src="http://jsxgraph.uni-bayreuth.de/distrib/prototype.js"></script>
<script type="text/javascript" src="http://jsxgraph.uni-bayreuth.de/distrib/loadjsxgraphXML.js"></script>
<script type="text/javascript" src="http://jsxgraph.uni-bayreuth.de/distrib/jsxgraphcore.js"></script>
<div style="width:960px">
<div id="jxgbox" class="jxgbox" style="width:600px; height:400px; float:left; "></div>
<p style="float:right">
<input type="text" id="eingabe" value="Math.sin(x)*Math.cos(x)">

<input type="button" value="set" onClick="doIt()" style='margin:1em'> 
</p>
</div>
<br clear=all>
<div id="debug" style="display:none;"></div>
   <script type="text/javascript">
    /* <![CDATA[ */

        board = JXG.JSXGraph.initBoard('jxgbox', {originX: 250, originY: 250, unitX: 40, unitY: 20});
		// Axes
        b1axisx = board.createElement('axis', [[1,0], [0,0]], {});
        b1axisy = board.createElement('axis', [[0,1], [0,0]], {});
		
		eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
		graph = board.createElement('curve', [function(x){ return x; }, function(x){ return f(x); }, "x", -10, 10]);						
		//graph = eval("board.createElement('curve', [function(x){ return x; }, function(x){ return "+document.getElementById("eingabe").value+";}, 'x', -10, 10])");
		//graph.curveType = "graph";
        p1 = board.createElement('glider', [graph], {style:6, name:'P'});
        p2 = board.createElement('point', [function() { return p1.X()+1;}, function() {return p1.Y()+board.algebra.D(graph.Y)(p1.X());}], {style:1, name:''});
        l1 = board.createElement('line', [p1,p2],{});
        p3 = board.createElement('point', [function() { return p2.X();}, function() {return p1.Y();}],{style:1, name:''});
        pol = board.createElement('polygon', [p1,p2,p3], {});
        t = board.createElement('text', [function(){return p1.X()+1.1;},function(){return p1.Y()+(p2.Y()-p3.Y())/2;},function(){ return "m="+(board.round(p2.Y()-p3.Y(),2));}]); 
            
        
        function doIt(){
        	eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
        	graph.yterm = function(x){ return f(x); };  // usually: e.g. "x^2"
			//graph.yterm = eval("function(x){ return "+document.getElementById("eingabe").value+"; };");
    		graph.Y = graph.yterm;
    		graph.updateCurve();
    		board.update();
        } 
        
  /* ]]> */
  </script>
</html>


== HowTo Create this Construction ==

=== HTML Part ===
'''Adding a text input field somewhere on the page together with a button'''

<source>
<input type="text" id="eingabe" value="Math.sin(x)*Math.cos(x)">
<input type="button" value="set" onClick="doIt()"> 
</source>

=== JavaScript Part ===
'''Setting up the board'''
<source>
  board = JXG.JSXGraph.initBoard('jxgbox', {originX: 250, originY: 250, unitX: 40, unitY: 20});
  // Axes
  b1axisx = board.createElement('axis', [[1,0], [0,0]], {});
  b1axisy = board.createElement('axis', [[0,1], [0,0]], {});
</source>

'''Create a JavaScript Function f(x) From the Text Field'''
<source>
  eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
</source>

'''Use f(x) for defining the Graph'''
<source>
graph = board.createElement('curve', [function(x){ return x; }, function(x){ return f(x); }, "x", -10, 10]);
</source>

'''Define Further Elements'''
<source>
  //glider on the curve
  p1 = board.createElement('glider', [graph], {style:6, name:'P'});
  //define the derivative of f
  g = board.algebra.D(f);
  //a point on the tangent
  p2 = board.createElement('point', [function() { return p1.X()+1;}, function() {return p1.Y()+board.algebra.D(graph.Y)(p1.X());}], {style:1, name:''});
  //the tangent 
  l1 = board.createElement('line', [p1,p2],{}); 
  //a third point fpr the slope triangle
  p3 = board.createElement('point', [function() { return p2.X();}, function() {return p1.Y();}],{style:1, name:''});
  //the slope triangle
  pol = board.createElement('polygon', [p1,p2,p3], {});
  //a text for displaying slope's value
  t = board.createElement('text', [function(){return p1.X()+1.1;},function(){return p1.Y()+(p2.Y()-p3.Y())/2;},function(){ return "m="+(board.round(p2.Y()-p3.Y(),2));}]); 
</source>

'''Define JavaScript Function doIt() for Reacting on User Input'''
<source>
  function doIt(){
    //redefine function f according to the current text field value
    eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
    //change the yterm attribute of the graph to the new function 
    graph.yterm = function(x){ return f(x); };  // usually: e.g. "x^2"
    graph.Y = graph.yterm;
    //update the graph
    graph.updateCurve();
    //update the whole board
    board.update();
  } 
</source>

== Remarks ==
The