要使用外部自定义按钮而不是图例来切换系列，可以使用amCharts的事件处理功能。以下是一个使用外部按钮切换系列的示例代码：

HTML代码：

系列1
系列2
系列3

JavaScript代码：

// 创建图表
var chart = am4core.create("chartdiv", am4charts.XYChart);

// 添加数据
chart.data = [{
  "category": "一月",
  "value1": 100,
  "value2": 200,
  "value3": 300
}, {
  "category": "二月",
  "value1": 150,
  "value2": 250,
  "value3": 350
}, {
  "category": "三月",
  "value1": 200,
  "value2": 300,
  "value3": 400
}];

// 创建类别轴
var categoryAxis = chart.xAxes.push(new am4charts.CategoryAxis());
categoryAxis.dataFields.category = "category";

// 创建值轴
var valueAxis = chart.yAxes.push(new am4charts.ValueAxis());

// 创建系列1
var series1 = chart.series.push(new am4charts.ColumnSeries());
series1.dataFields.valueY = "value1";
series1.dataFields.categoryX = "category";
series1.name = "Series 1";

// 创建系列2
var series2 = chart.series.push(new am4charts.ColumnSeries());
series2.dataFields.valueY = "value2";
series2.dataFields.categoryX = "category";
series2.name = "Series 2";

// 创建系列3
var series3 = chart.series.push(new am4charts.ColumnSeries());
series3.dataFields.valueY = "value3";
series3.dataFields.categoryX = "category";
series3.name = "Series 3";

// 隐藏图例
chart.legend = new am4charts.Legend();

// 外部按钮事件处理
document.getElementById("series1").addEventListener("click", function() {
  series1.hidden = !series1.hidden;
});

document.getElementById("series2").addEventListener("click", function() {
  series2.hidden = !series2.hidden;
});

document.getElementById("series3").addEventListener("click", function() {
  series3.hidden = !series3.hidden;
});

这段代码首先创建了一个包含三个系列的柱状图，并将数据添加到图表中。然后，通过addEventListener函数将点击事件添加到每个按钮上。当按钮被点击时，切换相应系列的hidden属性，从而显示或隐藏该系列。