canvas.js

// http://webreflection.blogspot.com/2009/01/ellipse-and-circle-for-canvas-2d.html
(function(){
  // Andrea Giammarchi - Mit Style License
  var extend = {
    // Circle methods
    circle:function(aX, aY, aDiameter){
      this.ellipse(aX, aY, aDiameter, aDiameter);
    },
    fillCircle:function(aX, aY, aDiameter){
      this.beginPath();
      this.circle(aX, aY, aDiameter);
      this.fill();
    },
    strokeCircle:function(aX, aY, aDiameter){
      this.beginPath();
      this.circle(aX, aY, aDiameter);
      this.stroke();
    },
    // Ellipse methods
    ellipse:function(aX, aY, aWidth, aHeight){
      aX -= aWidth / 2;
      aY -= aHeight / 2;
      var hB = (aWidth / 2) * .5522848,
      vB = (aHeight / 2) * .5522848,
      eX = aX + aWidth,
      eY = aY + aHeight,
      mX = aX + aWidth / 2,
      mY = aY + aHeight / 2;
      this.moveTo(aX, mY);
      this.bezierCurveTo(aX, mY - vB, mX - hB, aY, mX, aY);
      this.bezierCurveTo(mX + hB, aY, eX, mY - vB, eX, mY);
      this.bezierCurveTo(eX, mY + vB, mX + hB, eY, mX, eY);
      this.bezierCurveTo(mX - hB, eY, aX, mY + vB, aX, mY);
      this.closePath();
    },
    fillEllipse:function(aX, aY, aWidth, aHeight){
      this.beginPath();
      this.ellipse(aX, aY, aWidth, aHeight);
      this.fill();
    },
    strokeEllipse:function(aX, aY, aWidth, aHeight){
      this.beginPath();
      this.ellipse(aX, aY, aWidth, aHeight);
      this.stroke();
    }
  };

  for(var key in extend)
    CanvasRenderingContext2D.prototype[key] = extend[key];
})();