<svg	xmlns="http://www.w3.org/2000/svg" width="100%"
		xmlns:xlink="http://www.w3.org/1999/xlink" >
  <g id="sketch" class="sketch">

    <rect x="0"
y="0" width="100%" height="100%" fill="yellow" />
  </g>

  <script>
  <![CDATA[
  var	xmlns="http://www.w3.org/2000/svg"
  Root=document.documentElement
  on_start(Root)

  function mouseUp(evt) {
	  on_pen_up(Root);
  }

  function on_start(R){
	  var Attr={
	    "onmousemove":"mouseMove",
      "onmousedown":"add_line(evt)",	
      "onmouseup":"mouseUp"
	  }
	  assignAttr(R,Attr)
  }

  function on_pen_down(R){
	  var Attr={
	    "onmousemove":"addPoint(evt)",
	    "onmouseup":"on_pen_up(Root)"
	  }
	  assignAttr(R,Attr)
  }

  function on_pen_up(R){
	  var Attr={
	    "onmousemove":"null"
	  }
	  assignAttr(R,Attr)
  }

  function add_line(evt){
	  //if (evt.target.nodeName!="rect") return
	  var C=document.createElementNS(xmlns,"polyline") 
	
	  var Attr={
		  "x":30,
		  "y":180,
		  "fill":'none',
		  "stroke": '#44a',
		  "id":'pl1',
		  "stroke-width":2
	  }
    
	  assignAttr(C,Attr)
	  Root.appendChild(C)
    
	  on_pen_down(Root)
  }

  function addPoint (evt) {
    element = document.getElementById('pl1') 	
    var point = element.ownerDocument.documentElement.createSVGPoint(); 
    point.x = evt.clientX; 
    point.y = evt.clientY;
    
    element.points.appendItem(point);
  }

  function assignAttr(O,A){
	  for (i in A) O.setAttributeNS(null,i, A[i])
  }

  ]]>
  </script>

  <text font-size="12pt" x="50" y="20" id="t1">Click something to move it</text>
  <text font-size="12pt" x="80" y="40" id="t2">Click nothing to add something</text>
</svg>

<?xml version="1.0" encoding="UTF-8"?> 
<svg xmlns="http://www.w3.org/2000/svg" version="1.1"
 onclick="addPoint(evt, document.getElementById('pl1'));"> 
  <title>click to add point to polyline</title>
  <script type="text/ecmascript">
    <![CDATA[ 
      function addPoint (evt, element) {
        var point = element.ownerDocument.documentElement.createSVGPoint(); 
        point.x = evt.clientX; 
        point.y = evt.clientY; 
        element.points.appendItem(point);
      }
    ]]>
  </script> 
  <rect x="0" y="0" width="100%" height="100%" fill-opacity="0" />
  <polyline id="pl1" fill="none" stroke="green" stroke-width="2" points="30,180 50,150" />
</svg> 

//+ Jonas Raoni Soares Silva
//@ http://jsfromhell.com/math/closest-polyline-point [v1.0]

closestPolyLinePoint = function( px, py, x0, y0, x1, y1, etc, etc, etc ){
	function dotLineLength( x, y, x0, y0, x1, y1, o ){
		function lineLength( x, y, x0, y0 ){
			return Math.sqrt( ( x -= x0 ) * x + ( y -= y0 ) * y );
		}
		if( o && !( o = function( x, y, x0, y0, x1, y1 ){
			if( !( x1 - x0 ) ) return { x: x0, y: y };
			else if( !( y1 - y0 ) ) return { x: x, y: y0 };
			var left, tg = -1 / ( ( y1 - y0 ) / ( x1 - x0 ) );
			return { x: left = ( x1 * ( x * tg - y + y0 ) + x0 * ( x * - tg + y - y1 ) ) / ( tg * ( x1 - x0 ) + y0 - y1 ), y: tg * left - tg * x + y };
		}( x, y, x0, y0, x1, y1 ), o.x >= Math.min( x0, x1 ) && o.x <= Math.max( x0, x1 ) && o.y >= Math.min( y0, y1 ) && o.y <= Math.max( y0, y1 ) ) ){
			var l1 = lineLength( x, y, x0, y0 ), l2 = lineLength( x, y, x1, y1 );
			return l1 > l2 ? l2 : l1;
		}
		else {
			var a = y0 - y1, b = x1 - x0, c = x0 * y1 - y0 * x1;
			return Math.abs( a * x + b * y + c ) / Math.sqrt( a * a + b * b );
		}
	};
	for( var args = [].slice.call( arguments, 0 ), lines = []; args.length > 4; lines[lines.length] = { y1: args.pop(), x1: args.pop(), y0: args.pop(), x0: args.pop() } );
	if( !lines.length )
		return { x: px, y: py };
	var l, i, o = lines[0], lower = { i: 0, l: dotLineLength( px, py, o.x0, o.y0, o.x1, o.y1 ) };
	for( i in lines )
		if( lower.l > ( l = dotLineLength( px, py, ( o = lines[i] ).x0, o.y0, o.x1, o.y1 ) ) )
			lower = { i: i, l: l };
	py = py < Math.min( ( o = lines[lower.i] ).y0, o.y1 ) ? Math.min( o.y0, o.y1 ) : py > Math.max( o.y0, o.y1 ) ? Math.max( o.y0, o.y1 ) : py;
	px = px < Math.min( o.x0, o.x1 ) ? Math.min( o.x0, o.x1 ) : px > Math.max( o.x0, o.x1 ) ? Math.max( o.x0, o.x1 ) : px;
	return Math.abs( o.x0 - o.x1 ) < Math.abs( o.y0 - o.y1 ) ? { x: ( py * ( o.x0 - o.x1 ) - o.x0 * o.y1 + o.y0 * o.x1 ) / ( o.y0 - o.y1 ), y: py }
	: { x: px, y: ( px * ( o.y0 - o.y1 ) - o.y0 * o.x1 + o.x0 * o.y1 ) / ( o.x0 - o.x1 ) };
};