function getCanvasImage(_ctx, _bgColor){
    var tmpCtx = document.createElement("canvas").getContext("2d");
        tmpCtx.canvas.width  = _ctx.canvas.width;
        tmpCtx.canvas.height = _ctx.canvas.height;
        tmpCtx.fillStyle = _bgColor;
        tmpCtx.fillRect(0, 0, tmpCtx.canvas.width, tmpCtx.canvas.height);
        tmpCtx.drawImage(_ctx.canvas, 0, 0);

    return tmpCtx.canvas;
  }

// ...
// just call the function passing your canvas context and the background color
var graphImage = getCanvasImage(ctx, "#222");