            i -= STEP_Y;
            }

            // Draw vertical line
            context.strokeStyle = 'rgba(100,0,0,0.3)';
            context.lineWidth = 1;

            context.beginPath();
            context.moveTo(LEFT_MARGIN, 0);
            context.lineTo(LEFT_MARGIN, context.canvas.height);
            context.stroke();

            context.restore();

    };

    /**
     * 画渐变文本
     */
    let drawGradientText=()=>{
        context.fillStyle=gradient;
        context.fillText(text