How to customize the areaStyle? like this: `areaStyle:{ normal: { color: new echarts.graphic.LinearGradient(0, 0, 0, 1, [{ offset: 0, color: 'rgba(40, 182, 252, 0.85)' }, { offset: 1, color: 'rgba(28, 159, 255, 0.2)' }]) } }`