使用visualMap 来为单个柱子指定样式echarts 柱状配置项内容和展示

使用visualMap 来为不同category的单个柱子指定样式，应该怎么做，应该怎么配置

      var option = {
    dataset: {
        source: [
            ['score', 'amount', 'product'],
            [89.3, 58212, 'Matcha'],
            [57.1, 78254, 'Milk'],
            [74.4, 41032, 'Cocoa'],
            [50.1, 12755, 'Brownie'],
            [89.7, 20145, 'HCocoa']
        ]
    },
    xAxis: {},
    yAxis: {
        type: 'category'
    },
    visualMap: [{
        type: 'piecewise',
        dimension: 2,
        categories: ['Matcha', 'Milk', 'Cocoa', 'Brownie', 'HCocoa'],
        color: ['red', '#bf444c', '#d88273', '#f6efa6', 'blue']
    }],
    series: [{
        type: 'bar',
        encode: {
            x: 'score',
            y: 'product'
        }
    }]
};