Using Images

1. Create a new Image or use one from document.images.
2. Use drawImage to draw it on the canvas.

function drawchart() {
    var ctx = document.getElementById('canvas').getContext('2d');
    var img = new Image();
    img.src = 'backdrop.png';
    img.onload = function(){
        ctx.drawImage(img,0,0);
        ctx.beginPath();
        ctx.moveTo(30,96);
        ctx.lineTo(70,66);
        ctx.lineTo(103,76);
        ctx.lineTo(170,15);
        ctx.stroke();
    }
}

drawchart()

• Scale with drawImage(image, xpos, ypos, width, height)
• Slice with drawImage(image, sourcex, sourcey, sourcewidth, sourceheight, destx, desty, destwidth, destheight).