how to animate drawing lines on canvas

I understand that your want the lines to extend incrementally along the points in your path using animation.

A Demo: http://jsfiddle.net/m1erickson/7faRQ/

You can use this function to calculate waypoints along the path:

// define the path to plot
var vertices=[];
vertices.push({x:0,y:0});
vertices.push({x:300,y:100});
vertices.push({x:80,y:200});
vertices.push({x:10,y:100});
vertices.push({x:0,y:0});

// calc waypoints traveling along vertices
function calcWaypoints(vertices){
    var waypoints=[];
    for(var i=1;i<vertices.length;i++){
        var pt0=vertices[i-1];
        var pt1=vertices[i];
        var dx=pt1.x-pt0.x;
        var dy=pt1.y-pt0.y;
        for(var j=0;j<100;j++){
            var x=pt0.x+dx*j/100;
            var y=pt0.y+dy*j/100;
            waypoints.push({x:x,y:y});
        }
    }
    return(waypoints);
}

Then you can use requestAnimationFrame to animate each incremental line segment:

// calculate incremental points along the path
var points=calcWaypoints(vertices);


// variable to hold how many frames have elapsed in the animation
// t represents each waypoint along the path and is incremented in the animation loop
var t=1;


// start the animation
animate();

// incrementally draw additional line segments along the path
function animate(){
    if(t<points.length-1){ requestAnimationFrame(animate); }
    // draw a line segment from the last waypoint
    // to the current waypoint
    ctx.beginPath();
    ctx.moveTo(points[t-1].x,points[t-1].y);
    ctx.lineTo(points[t].x,points[t].y);
    ctx.stroke();
    // increment "t" to get the next waypoint
    t++;
}