HTML5 Canvas Scale a Drawing with Plus and Minus Buttons

function draw(scale, translatePos){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
 
    // clear canvas
    context.clearRect(0, 0, canvas.width, canvas.height);
 
    context.save();
    context.translate(translatePos.x, translatePos.y);
    context.scale(scale, scale);
    context.beginPath(); // begin custom shape
    context.moveTo(-119, -20);
    context.bezierCurveTo(-159, 0, -159, 50, -59, 50);
    context.bezierCurveTo(-39, 80, 31, 80, 51, 50);
    context.bezierCurveTo(131, 50, 131, 20, 101, 0);
    context.bezierCurveTo(141, -60, 81, -70, 51, -50);
    context.bezierCurveTo(31, -95, -39, -80, -39, -50);
    context.bezierCurveTo(-89, -95, -139, -80, -119, -20);
    context.closePath(); // complete custom shape
    var grd = context.createLinearGradient(-59, -100, 81, 100);
    grd.addColorStop(0, "#8ED6FF"); // light blue
    grd.addColorStop(1, "#004CB3"); // dark blue
    context.fillStyle = grd;
    context.fill();
 
    context.lineWidth = 5;
    context.strokeStyle = "#0000ff";
    context.stroke();
    context.restore();
}
 
window.onload = function(){
    var canvas = document.getElementById("myCanvas");
 
    var translatePos = {
        x: canvas.width / 2,
        y: canvas.height / 2
    };
 
    var scale = 1.0;
    var scaleMultiplier = 0.8;
    var startDragOffset = {};
    var mouseDown = false;
 
    // add button event listeners
    document.getElementById("plus").addEventListener("click", function(){
        scale /= scaleMultiplier;
        draw(scale, translatePos);
    }, false);
 
    document.getElementById("minus").addEventListener("click", function(){
        scale *= scaleMultiplier;
        draw(scale, translatePos);
    }, false);
 
    // add event listeners to handle screen drag
    canvas.addEventListener("mousedown", function(evt){
        mouseDown = true;
        startDragOffset.x = evt.clientX - translatePos.x;
        startDragOffset.y = evt.clientY - translatePos.y;
    });
 
    canvas.addEventListener("mouseup", function(evt){
        mouseDown = false;
    });
 
    canvas.addEventListener("mouseover", function(evt){
        mouseDown = false;
    });
 
    canvas.addEventListener("mouseout", function(evt){
        mouseDown = false;
    });
 
    canvas.addEventListener("mousemove", function(evt){
        if (mouseDown) {
            translatePos.x = evt.clientX - startDragOffset.x;
            translatePos.y = evt.clientY - startDragOffset.y;
            draw(scale, translatePos);
        }
    });
 
    draw(scale, translatePos);
};