Incubator-echarts: 折线图lineStyle中的shadowColor不支持渐变色

[dtzh420]

Version

4.5.0

Steps to reproduce

在echarts配置项手册上看到lineStyle里面的shadowColor支持的格式跟color一致，那么就意味着是可以支持渐变色的。但是我这边使用下面的配置之后发现，线的颜色渐变了，但是阴影没有展示，把shadowColor的颜色换成rgba格式之后，阴影就可以正常显示了。

option = {
    xAxis: {
        type: 'category',
        data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    },
    yAxis: {
        type: 'value'
    },
    series: [{
        data: [820, 932, 901, 934, 1290, 1330, 1320],
        type: 'line',
        smooth: true,
        lineStyle: {
            color: {
                type: 'linear',
                x: 0,
                y: 0,
                x2: 0,
                y2: 1,
                colorStops: [{
                    offset: 0, color: 'red' // 0% 处的颜色
                }, {
                    offset: 1, color: 'blue' // 100% 处的颜色
                }],
                global: false // 缺省为 false
            },
            shadowColor: {
                type: 'linear',
                x: 0,
                y: 0,
                x2: 0,
                y2: 1,
                colorStops: [{
                    offset: 0, color: 'red' // 0% 处的颜色
                }, {
                    offset: 1, color: 'blue' // 100% 处的颜色
                }],
                global: false // 缺省为 false
            },
            shadowBlur: 10,
            shadowOffsetY: 12
        }
    }]
};

What is expected?

阴影正常出现，并且颜色是渐变的。

What is actually happening?

阴影没有出现

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[alex2wong]

shadowColor 应该不支持渐变，文档可能需要更新下。因为底层 canvas 如果直接采用ctx.shadowColor 去配置，是不支持 LieanerGradient 对象的。